Fundamentals of Electrical Transmission and Distribution--Drawings and Diagrams

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This chapter describes different types of electrical diagrams. It explains how the diagrams are developed from original concepts into drawings that describe the full operation of the system and how further drawings, schedules and diagrams are produced in order to enable the system to be constructed at the factories, installed, tested and commissioned on site. Examples are given of a variety of different styles of presentation based upon manufacturers and National Standards. This chapter concentrates on substation control and protection schemes, but the principles apply equally to most electrical plant.


The starting point for new substation work is the block diagram or single line diagram (SLD). A typical example is given in FIG. 1. The various elements (current transformers (CTs), meters, control and relay equipment, etc.) are shown symbolically superimposed upon the substation SLD. FIG. 1 uses symbols based upon international practice.

FIG. 1 Substation block diagram.

The advantage of this type of diagram is that the complete system can be seen as a whole in semi-pictorial form. Although not meant to be a detailed guide for the layout of the controls and instruments on the control panels, it is sufficiently concise to enable the designer to check that all the facilities required by the operator are present. Similarly for the relay cubicle, the block diagram only illustrates the general requirements for the siting of the relays. A single item on the block diagram could, for example, represent a complex relay scheme which in itself could occupy several racks on the protection panel.

The correct location of CTs for different functions, the summation of CT windings, overlapping of protection zones, selection of voltage transformers (VTs), etc., can all be easily checked on the block diagram. Location of such items as auto-reclose on/off switches (which could be mounted on the control or relay panel or elsewhere) can be seen. Details such as CT ratios, current ratings of switchgear, etc., are also included on these drawings.

The block diagram is usually included in the contract document as part of a tender specification for new works. A fundamental requirement in any such documentation is for the engineer specifying the equipment to leave no doubt as to the exact requirements. The block diagram is therefore usually completed before the scheduling of equipment since its pictorial representation makes it easy to visualize the completed equipment and therefore no major item is missed or wrongly placed. Lack of definition at the tender stage will only result in claims for variations to the contract and extra costs later by the contractor. In particular the need to define local and remote alarms, metering and control may be further clarified using the form of protection block diagram detailed in FIG. 2.

Three line diagrams are similar to single line diagrams but show all three phases. This added detail gives further information and is sometimes useful to assist in the location of VTs and especially to help describe three phase to single phase traction substation schemes.


3.1 Method of Representation

Schematic diagrams describe the main and auxiliary circuits for control, signaling, monitoring and protection systems. They are drawn in sufficient detail to explain to the user the circuitry and its mode of operation. They allow circuits to be 'followed through' when tracing faults.

With the increasing complexity of electronic circuitry, discrete functional blocks, such as relays, are often represented as 'black boxes' on the overall schematic diagrams with only the input and output terminals to these units clearly identified. The characteristics of the 'black boxes' are identified by standard symbols and further reference to the complex circuitry inside the 'black boxes' may be obtained from separate manufacturers' drawings. Circuit arrangements are usually shown on schematic diagrams according to their functional aspects.

They seldom follow the actual physical layouts of the different component parts.

The key to understanding schematic diagrams is that the switchgear units and relay contacts on the diagrams are shown in their non-energized or standard reference condition. For example, normally open (NO) and normally closed (NC) contacts are shown in their open or closed states respectively.

Any exceptions to this rule must be clearly marked on the diagram.


FIG. 2 Protection block diagram.

Notes :

1. Balanced earth fault relay

2. Restricted earth fault relay

3. Standby earth fault relay

4. Tap changer blocking relay included in AVR scheme

5. Master trip relays to be incorporated on 132 kV and 20 kV CBs.


FIG. 3 Typical main circuit diagram motor starter circuit.

3.2 Main Circuits

Main circuits are shown in their full three phase representation as indicated in the motor starter circuit of FIG. 3. The diagrams are drawn from power source (top) to load (bottom) showing the main power connections, switching and protection arrangements.

3.3 Control, Signaling and Monitoring Circuits

European practice is to draw circuits between horizontal potential lines (positive at top and negative at bottom for DC control). The signal and information flow in open-loop control systems is also generally from the top of the drawing down and for closed-loop systems from left to right. FIG. 4 shows a typical control schematic arising from a motor starter system. Relay auxiliary contacts (in their standard reference condition) are shown beneath each relay coil. Exact codification varies from manufacturer to manufacturer as explained in Section 4. Some examples of European practice are given in FIG. 5a to e.

FIG. 4 Typical control schematic (associated with FIG. 3 motor circuit).

FIG. 5 Equipment markings -- European practice: (a) NC contact with the terminals 51 and 52 of the contactor -- K12 shown in the schematic diagram in the resolved state; (b) terminal markings shown at the complete equipment representation (contactor); (c) markings of three terminals on terminal block 2X1; (d) method for representing wiring; (e) partial representation of an item of equipment in different diagrams


Manufacturers use different styles and presentations for their drawings. This section compares some of the advantages and disadvantages of different manufacturers' styles by comparing a simple control and protection scheme for a feeder circuit. The concept block diagram for this feeder is shown in FIG. 6. For simplicity in all these examples the CT connections have not been shown.

FIG. 6 Concept block diagram (simple feeder circuit).

4.1 Combined Wiring/Cabling Diagrams

FIG. 7 illustrates this type of drawing. Usually drawn on a large sheet of A0 or A1 size paper it shows the rear view of all equipments in the control and relay cubicles, the equipment in the circuit breaker and the multicore cable interconnections. The internal arrangements of the contacts in switches and relays are shown. In practice, the drawing would also show CT connections, relay coils, etc., which would make the circuits even more difficult to follow.

This type of drawing dates back to the days when control switches were mounted on flat slabs of stone or slate supported on a steel frame. (This is where the term 'panel' originates and it is still used by many engineers to refer to a relay or control cubicle.) It is found on circuits for older, relatively simple equipment.

The advantage of this type of drawing is that it is possible to trace the complete circuit of part of a control scheme from only one diagram. It is a pictorial representation of the physical wiring layout and may be used by the electrician or wireman at the manufacturer's works to install the wires in the cubicles, used by the site staff to install and terminate multicore cables, by commissioning staff to check operation and testing and also later by operating staff for fault tracing or future modifications. For site use the wires are often colored by operations staff to assist in circuit tracing.

A major disadvantage of this type of drawing is that it is difficult to trace all branches and sub-branches of complex control and relay circuits. When the large format drawing has been folded and unfolded many times it becomes difficult to follow the individual wires as they cross or follow the drawing creases. The advent of more complex protection schemes and multicore electronic data lines has meant that drawings of this type are rarely produced today.

FIG. 7 Combined schematic wiring cabling diagram for 11 kV circuit breaker feeder.

4.2 British Practice

Although traditional, older British practice is described here, the same basic system has also been used in the United States, Japan and other countries.

Working straight from the block diagram, two separate drawings are produced for each HV circuit showing VT/CT circuits in one and DC/auxiliary AC circuits in the other. Large A0 or A1 sheets of paper are employed. VT and CT circuits are fully drawn out showing schematically every CT wire, tapping, relay coil, terminals, etc. The second drawing has the complete DC control system drawn out schematically from 1ve to _ve, left to right. The DC circuit diagram also usually includes any auxiliary AC circuits (for heaters, motors, etc.). FIG. 8 shows the DC schematic for a simple distribution voltage level feeder circuit.

FIG. 8 Traditional British style DC schematic diagram.

Each circuit (closing circuit, trip circuit, etc.) is clearly shown, each branch can be traced and isolation points such as fuses or links are indicated.

Some manufacturers put on terminal markings for different types of equipment (as shown in FIG. 8); others draw dotted lines around the limits of control cubicles, protection cubicles, etc.

The principal difference between the traditional British practice and most other systems is that wires in the British system are identified by a wire number and not by a terminal number. The wire number has an alphabetic prefix which identifies the type of circuit. For example, trip circuit (K), alarm circuit (L), CT circuit (A, B or C, depending on the type of protection), etc. The number identifies the relative position in the circuit. The number changes at each contact, coil, indicator lamp, etc., but the same number is used for all branches of a circuit which are directly connected together.

Most British manufacturers write a full description of the protection relay or an abbreviation (e.g. 23O/C, 13E/F IDMT for two-pole overcurrent and one pole earth fault inverse definite minimum time-lag relay) next to the unit together with the manufacturer's type number. There is also a numerical code system for relay identification widely used by the US and Japanese manufacturers. A list is given in SECTION A.

FIG. 9 shows the type of drawing produced by the manufacturer to enable the wiring to be carried out in the works. Sometimes the multicore cable connections are also shown on the same diagram. The equipment and terminals blocks are drawn as seen by the wireman from the back of the cubicle and is very useful to assist the site staff during fault tracing, service, and for future modifications. Similar drawings are supplied for all relay and control cubicles, marshalling kiosks, circuit breakers, etc.

Multicore cable schedules are also supplied by the manufacturers. These schedules give the number of cores required, the wire numbers to be given to the cores, types of cable, lengths, sizes, etc.

FIG. 9 Cubicle wiring diagram (British traditional practice).

4.3 European Practice

Many European manufacturers used to produce schematic diagrams on paper 30 cm high (A4 height) but in fold-out format to be as long as necessary (often several meters long) to show all the circuits. This type of drawing produced on rolls of tracing paper could be reproduced easily in long lengths using dye line printers. With the advent of computer aided drafting or design (CAD) and cheap electrostatic photocopying machines most European manufacturers now produce a series of A3 format schematic drawings. Some manufacturers break down their large drawings into several A3 sheets based upon 'functional' aspects of the circuitry and some produce 'unit'-orientated drawings.

In the 'function' drawings, one sheet (or group of sheets) will cover a complete circuit such as a circuit breaker and isolator interlocking scheme, indication and alarm scheme, CT circuit, etc.

FIG. 10a DC supplies.

FIG. 10b Control and indication.

FIG. 10c 150 kV Line overcurrent protection.

FIG. 10d Circuit breaker.

In the 'unit'-orientated drawings, one or more drawings will show an item of plant and all parts of all circuits wired through that plant. For example, the drawings showing a circuit breaker will include part of the trip, close, synchronizing, alarms, interlocking supervisory functions, etc.

Manufacturers using these types of diagrams have developed their own codes for identification of equipment function and location. They are all generally based upon the German DIN standard. Their starting point for producing drawings is to take the concept block diagram and produce further block diagrams for each HV circuit using their own symbols and terminology. Schedules are also provided to explain to the uninitiated the meaning of the various symbols and codes used.

Figures 10a to d illustrate the type of diagrams produced by ABB (previously known as ASEA Brown Boveri) from the block diagram of FIG. 6.

These are basically 'unit' rather than 'function' orientated.

FIG. 10a shows the DC supply sources for the various trip, close and indications. On a more complex scheme these may be on separate sheets.

Incoming supplies are shown on the lower side of the dashed line indicating the boundary of the relay cubicle, RP. The symbols R1 ,R_, etc., shown on the wires do not physically exist on the wires themselves. They are added on the schematics to enable the same wire to be identified on the continuation sheets (Figs. 10b to d). Looped connections for the 1ve and _ve lines within the cubicles are not shown to avoid cluttering the diagram. It is there fore not possible to tell from the diagram if the 1ve wire from terminal B50.2 loops first to switch F1 terminal 3, switch F2 terminal 3 or switch F3 terminal 3. For this information you would have to consult the separate wiring schedule. The D29 indicates that all the items in the small dotted rectangle (F1, F2 and F3) are mounted in location D29. One firm's practice is to designate the terminal block for power supplies, etc., in any cubicle as B50, even if the miniature circuit breakers are mounted at location 1D29.

FIG. 10b shows some of the circuits on the control panel. Again incoming supplies are at the lower edge of the dotted line representing the boundary of the control panel. The 'close', 'main trip', etc., descriptions on the wires from terminals X1.4, X1.5, etc., are to enable the continuation of the same circuits to be identified on FIG. 10d.

FIG. 10c shows in block form the protective relays and connections.

The 74310027-EB refers to a separate drawing which shows the full internal wiring of the relay. In some cases the manufacturers might not be willing to reveal the full details of their electronic 'black boxes'. The 1D13 refers to the location of the relay within the relay cubicle 1RP. For simplicity, the CT circuits have not been included in the drawing but in practice they would continue on one or more other sheets.

FIG. 10d shows the circuit breaker. As can be seen, the drawing includes trip, close and indication circuits.

The main disadvantage of this type of schematic diagram is that it is difficult to follow a complete circuit because it runs from page to page. Even with the simple example shown, to trace the working of the 'spring discharged' indication circuit requires reference to Figs. 10a, b and d, and back to FIG. 10a. On a double busbar substation, to trace out the workings of the interlocking scheme might need reference to say 20 drawings. The drawings are also difficult to modify since a single wire change may affect many diagrams and schedules. With this type of schematic diagram the manufacturer also produces a wiring table or schedule for all the wires between the terminal blocks, relays and other equipment in the cubicle, the inter-cubicle wiring, and to switchgear. When checking and commissioning equipment it is possible to miss a parallel circuit and it is sometimes useful to colour the diagrams as each branch of a circuit is tested and checked off.

The advantage of this type of drawing is that all the drawings are of a manageable size; they can be easily stored and handled at the work place using A3 files. Standard schemes can be worked up by the manufacturer and the diagrams assist the moves to European standardization.

4.4 Other Systems

Three basic systems have been described: the older all-in-one drawings which are still encountered on relatively simple schemes and may still be encountered in existing substations; the older British traditional system which is similar to the Japanese, US and other countries where large drawings are used and the more modern systems with numerous small (A3 and A4) drawings and schedules. It is not practical to compare drawings from every main manufacturer but variations and combinations of each of these types exist. The important thing is that whenever a system of drawings is encountered for the first time, the engineer concerned should ensure that the manufacturer supplies full details of the schemes employed together with lists of standard symbols and codes. This is necessary to ensure that the operating staff have sufficient information to operate and maintain the plant and update the drawings if future modifications are carried out. SECTION B gives a comparison between some German, British, US/Canadian and International symbols.


Most manufacturers now use computer aided design (CAD) to assist in the preparation of diagrams. The designer's basic sketch is used by the computer operator and standard symbols for relays, switches, etc., are called up from the computer memory files and placed in the appropriate positions on the visual display unit (VDU) representation of the drawing. The wiring interconnections are added, location details entered and when satisfactory a hard copy printout of the drawing is produced. At the same time by correct programming the computer can produce all the interconnection and multicore cable schedules directly from the drawing input. CAD is not only limited to A3 drawings but can be used to produce drawings in A0 sheet format using suitable printers or plotters. FIG. 2 is a typical example of a block diagram prepared using CAD with the relay symbols called up from a memory bank.

The main disadvantage with CAD is that the finished drawings may be more useful from the manufacturer's point of view since they will be difficult to update or modify on site without access to the original drawing data files. It is therefore usual to specify that final drawings are provided in electronic form as well as in hard copy.

Any disadvantage is outweighed by the clarity of the diagrams and circuit standardization which the control or protection engineer quickly recognizes.

Modifications to schematics, for example during commissioning, can now be carried out on site using a personal computer and issued back to head office electronically for updating the master copies. In addition the ability to 'layer' CAD drawings is especially useful. The 'background layer' file can contain the manufacturer's standard scheme so that customization of standard designs can be quickly achieved. Substation cable routing diagrams can be based upon the civil substation trench layout contained in one file layer and the cable routing design superimposed upon it by the designer in another layer. Strict drawing office procedures are necessary to ensure layers are not muddled. Another word of caution here is that when exporting your data exchange files (DXF) to another manufacturer or customer they will then have access to your symbol data bank which may have taken many hours to construct.


The block diagram shown in FIG. 1 has formed part of the tender documentation for the Greater New Town Transmission and Distribution Phase V Project.

The project is being sponsored by a large international aid agency which has appointed a firm of consulting engineers to prepare the contract documentation and administer the contract. At the present time assume that the contract has already been let on a 'turnkey basis' after competitive tender with one of five well-known design and construct contractors, namely S. U. B. Betabuilder Plc.

The client and substation operator's representative has called the consultant and contractor to a meeting at short notice because of a dispute regarding the substation works.

The client is refusing to accept the 'completed' works. The contractor is refusing to carry out further modifications to the metering alarms and indications associated with the protection and control scheme of the feeder circuits, although the threat of S. U. B. Betabuilder's performance bond not being released by the client is hanging over them. S. U. B. Betabuilder's line is that this is a 'turnkey contract' and that they have built the substation in accordance with the tender drawings. In addition all the substation detailed design drawings have been approved during the design process by the consultant. Further, the aid agency (which pays the consultant's fees) has got to hear of the dispute and is applying pressure on the consultant to resolve rapidly the issue 'in accordance with the terms and conditions of the contract'.

The meeting is strained and Mr Ali thumps the table and asks the consultant, 'Well, what did you exactly specify for the monitoring and control of the feeder circuits in this substation? My engineers tell me that additional work is necessary for interfacing with the Greater New Town Stage VI system control and data acquisition (SCADA) scheme, which is already out to tender. Are you expecting me to start issuing variation orders even before the SCADA contract is let?'

The consultant lays out on the table the substation block diagram ( FIG. 1) and the schedule of requirements from the tender document as shown in Table 2.1. 'It is quite clear what the requirements are. In any case any competent contractor would have enquired exactly what was required before entering the definitive detailed design stage.' Do you agree? How could the design definition, if required, have been improved in the tender documentation? Illustrate with a diagram for inclusion in a future tender document by the consultant if they get another job on the Greater New Town Project.

Do you think a variation order issued to S. U. B. Betabuilder would resolve the problem?


TABLE 1 Schedule of Requirements -- Overhead Line Protection Circuits

Item Description Quantity

1. Main protection -- distance relay 1

2. Direction earth -- fault relay (short lines only B20 km) 1

3. Backup protection -- IDMTL O/C and E/F elements 1

4. Intertripping Set

5. Auto-reclosing -- high-speed single pole (delayed auto-reclosing with check synchronizing where shown on the single line diagram) Set

6. All items necessary, whether fully described or not, are deemed to be included in the scope of the contract and will be checked during the course of the works Set



IEC 60617 is a very important reference for graphical symbols used in drawings and diagrams. It is divided into several parts, but is only now accessible through an online data base IEC 60617-DB-12M. CENELEC has therefore withdrawn the identical EN 60617 as a set of publications, on the basis that the symbols are more conveniently available from the IEC web site. The UK equivalent BSEN60617 (which superseded BS 3939) has been similarly treated. The useful BS 7845, a guide to 60617, is currently available from libraries but has been declared 'obsolescent'.

Part 7 of 60617 covering switchgear, control gear and protective devices is especially useful. Protection relays are drawn as a box (see FIG. 1) with a symbol describing its function. For example:

I > Delayed overcurrent relay with inverse time-lag characteristic Z < Under-impedance relay

Recommendations for printing symbols and numbers to represent electro magnetic quantities (volt, V, etc.) are described in IEC60027, 'Letter symbols to be used in electrical technology'.


In addition to the use of international symbols for indicating protection relay functions (see EN 60617-7, 1996) American ANSI Standard C37-2 numbering is also used. Where more detailed relay functionality is required this is achieved by supplementing the numbering system with letters. An underfrequency relay then has the ANSI coding of 81 U and an IEC 60617 symbol.

f < This appendix describes this numbering system and suffix letters.

Device number, definition and function:

1. Master element is the initiating device such as a control switch voltage relay float switch, etc., which serves either directly or through such permissive devices as protective and time-delay relays to place an equipment in or out of operation.

2. Time-delay starting, or closing, relay is a device which functions to give a desired amount of time delay before or after any point of operation in a switching sequence of protective relay system except as specifically provided by device functions 63 and 79 described later.

3. Checking or interlocking relay is a device which operates in response to the position of a number of other devices or to a number of predetermined conditions in an equipment to allow an operating sequence to proceed to stop or to provide a check of the position of these devices or of these conditions for any purpose.

4. Master contactor is a device generally controlled by device ... or equivalent and the necessary permissive and protective devices which serves to make and break the necessary control circuits to place an equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.

5. Stopping device functions to place and hold an equipment out of operation.

6. Starting circuit breaker is a device whose principal function is to connect a machine to its source or starting voltage.

7. Anode circuit breaker is one used in the anode circuits of a power rectifier for the primary purpose of interrupting the rectifier circuit if an arc-back should occur.

8. Control power disconnecting device is a disconnecting device -- such as a switch, circuit breaker or pullout fuse block -- used for the purpose of connecting and disconnecting, respectively the source of control power to and from the control bus or equipment.

Note: Control power is considered to include auxiliary power which supplies such apparatus as small motors and ... .

9. Reversing device is used for the purpose of reversing a machine held or for performing any other reversing functions.

10. Unit sequence switch is used to change the sequence in which units may be placed in and out of service in multiple unit equipments.

11. Reserved for future application.

12. Over-speed device is usually a direct-connected speed switch which functions on machine overspeed.

13. Synchronous-speed device, such as a centrifugal-speed switch, a slip frequency relay, a voltage relay, an undercurrent relay or any type of device, which operates at approximately synchronous speed of a machine.

14. Under-speed device functions when the speed of a machine falls below a predetermined value.

15. Speed or frequency, matching device functions to match and hold the speed or the frequency of a machine or of a system equal to or approximately equal to that of another machine source or system.

16. Reserved for future application.

17. Shunting, or discharge, switch serves to open or to close a shunting circuit around any piece of apparatus (except a resistor) such as a machine held, a machine armature, a capacitor or a rectifier.

Note: This excludes devices which perform such ... operations as may be necessary to the process of starting a machine by devices 6 or 42 or their equivalent ... excludes device 73 function which serves for the switching on resistors.

18. Accelerating or decelerating device as used to close or to cause the closing of circuits which are used to increase or to decrease the speed of a machine.

19. Starting-to-running transition contactor is a device which operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.

20. Electrically operated valve is a solenoid or motor-operated valve which is used in a vacuum, air, gas, oil, water or similar lines.

Note: The function of the valve may be indicated by the insertion of descriptive words such as Brake in Pressure Reducing in the function name, such as Electrically Operated Brake Valve.

21. Distance relay is a device which functions when the circuit admittance impedance or reactance increases or decreases beyond pre determined limits.

22. Equalizer circuit breaker is a breaker which serves to control or to make and break the equalizer or the current-balancing connections for a machine held, or for requesting equipment, in a multiple unit installation.

23. Temperature control device functions to raise or to lower the temperature of a machine or other apparatus, or of any medium when its temperature falls below, or rises above, a predetermined value.

Note: An example is a thermostat which switches on a space heater in a switchgear ... when the temperature falls to a desired value as distinguished from a device which is used to provide automatic temperature registering between close limits and would be designated as 90T.

24. Reserved for future application.

25. Synchronizing, or synchronism-check, device operates when two AC circuits are within the desired limits of frequency, phase angle or voltage to permit or to cause the paralleling of these two circuits.

26. Apparatus thermal device functions when the temperature of the shunt held or the armortisseur winding of a machine, or that of a load limiting or load shunting resistor or of a liquid or other medium exceeds a predetermined value or if the temperature of the protected apparatus such as a power rectifier or of any medium decreases below a predetermined value.

27. Undervoltage relay is a device which functions on a given value of undervoltage.

28. Reserved for future application.

29. Isolating contactor is used expressly for disconnecting one circuit from another for the purposes of emergency operating, maintenance or test.

30. Annunciator relay is a non-automatically reset device which gives a number of separate visual indications upon the functioning of protective devices, and which may also be arranged to perform a lock-out function.

31. Separate excitation device connects a circuit such as the shunt held of a synchronous converter to a source of separate excitation during the starting sequence or one which energizes the excitation and ignition circuits of a power rectifier.

32. Directional power relay is one which functions on a desired value of power flow in a given direction or upon reverse power resulting from arc-back in the anode or cathode circuits of a power rectifier.

33. Position switch makes or breaks contact when the main device or piece of apparatus which has no device function number reaches a given position.

34. Motor-operated sequence switch is a multi-contact switch which fixes the operating sequence of the major devices during starting and stopping, or during other sequential switching operations.

35. Brush-operating, or slip-ring short-circuiting, device is used for raising, lowering or shifting the brushes of a machine, or for short circuiting its slip rings, or for engaging or disengaging the contacts of a mechanical rectifier.

36. Polarity device operates or permits the operation of another device on a predetermined polarity only.

37. Undercurrent or underpower relay is a device which functions when the current or power flow decreases below a predetermined value.

38. Bearing protective device is one which functions on excessive bearing temperature or on other abnormal mechanical conditions, such as undue wear which may eventually result in excessive bearing temperature.

39. Reserved for future application.

40. Field relay is a device that functions on a given or abnormally low value or failure of machine-held current or on an excessive value of the reactive component or armature current in an arc machine indicating abnormally low-held excitation.

41. Field circuit breaker is a device which functions to apply or to remove the held excitation of a machine.

42. Running circuit breaker is a device whose principal function is to connect a machine to its source of running voltage after having been brought up to the desired speed on the starting connection.

43. Manual transfer or selector device transfers the control circuits so as to modify the plan of operation of the switching equipment or of some of the devices.

44. Unit sequence starting relay is a device which functions to start the next available unit in a multiple unit equipment on the failure or on the non-availability of the normally preceding unit.

45. Reserved for future application.

46. Reverse-phase, or phase-balance, current relay is a device which functions when the polyphase currents are of reverse-phase sequence or when the polyphase currents are unbalanced or contain negative phase-sequence components above a given amount.

47. Phase-sequence voltage relay is a device which functions upon a predetermined value of polyphase voltage in the desired phase sequence.

48. Incomplete sequence relay is a device which returns the equipment to the normal, or off, position and locks it out if the normal starting operating or stopping sequence is not properly completed within a predetermined time.

49. Machine, or transformer, thermal relay is a device which functions when the temperature of an AC machine armature, or of the armature or other load carrying winding or element of a DC machine or converter or power rectifier or power transformer (including a power rectifier transformer) exceeds a predetermined value.

50. Instantaneous overcurrent, or rate-of-rise, relay is a device which functions instantaneously on an excessive value of current, or on an excessive rate of current rise, thus indicating a fault in the apparatus or circuit being protected.

51. AC time overcurrent relay is a device with either a definite or inverse time characteristic, which functions when the current in an AC circuit exceeds a predetermined value.

52. AC circuit breaker is a device which is used to close and interrupt an AC power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

53. Exciter or DC generator relay is a device which forces the DC machine-held excitation to build up during starting or which functions when the machine voltage has built up to a given value.

54. High-speed DC circuit breaker is a circuit breaker which starts to reduce the current in the main circuit in 0.01 s or less, after the occurrence of the DC overcurrent or the excessive rate of current rise.

55. Power factor relay is a device which operates when the power factor in an AC circuit becomes above or below a predetermined value.

56. Field application relay is a device which automatically controls the application of the field excitation to an AC motor at some pre determined point in the slip cycle.

57. Short-circuiting or grounding device is a power or stored energy operated device which functions to short-circuit or to ground a circuit in response to automatic or manual means.

58. Power rectifier misfire relay is a device which functions if one or more of the power rectifier anodes fails to fire.

59. Overvoltage relay is a device which functions on a given value of overvoltage.

60. Voltage balance relay is a device which operates on a given difference in voltage between two circuits.

61. Current balance relay is a device which operates on a given difference in current input or output of two circuits.

62. Time-delay stopping, or opening, relay is a time-delay device which serves in conjunction with the device which initiated the shut down stopping, or opening, operation in an automatic sequence.

63. Liquid or gas pressure, level, or flow relay is a device which operates on given values of liquid or gas pressure, flow or level or on a given rate of change of these values.

64. Ground protective relay is a device which functions on failure of the insulation of a machine transformer or of the other apparatus to ground or on flashover of a DC machine to ground.

Note: This function is designated only to a relay which detects the flow of current from the frame of a machine or enclosing case or structure or a piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in the secondary circuit or secondary neutral of a current transformer or current transformers, connected in the power circuit or a normally grounded system.

65. Governor is the equipment which controls the gate or valve opening of a prime mover.

66. Notching, or jogging, device functions to allow only a specified number of operations of a given device, or equipment, or a specified number of successive operations within a given time of each other. It also functions to energize a circuit periodically, or which is used to permit intermittent acceleration or jogging of a machine at low speeds for mechanical positioning.

67. AC directional overcurrent relay is a device which functions on a desired value or AC overcurrent flowing in a predetermined direction.

68. Blocking relay is a device which initiates a pilot signal for blocking of tripping on external faults in a transmission line or in other apparatus under predetermined conditions, or co-operated with other devices to block tripping or to block reclosing on an out-of step condition or on power swings.

69. Permissive control device is generally a two-position, manually operated switch which in one position permits the closing of a circuit breaker, or the placing of an equipment into operation, and in the other position prevents the circuit breaker or the equipment from being operated.

70. Electrically operated rheostat is a rheostat which is used to vary the resistance of a circuit in response to some means of electrical control.

71. Reserved for future application.

72. DC circuit breaker is used to close and interrupt a DC power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

73. Load resistor contactor is used to shunt or insert a step of load limiting, shifting or indicating resistance in a power circuit, or to switch a space heater in circuit or to switch a light or regenerative, load resistor or a power rectifier or other machine in and out of circuit.

74. Alarm relay is a device other than an annunciator as covered under device No. 30 which is used to operate, or to operate in connection with a visual or audible alarm.

75. Position changing mechanism is the mechanism which is used for moving a removable circuit breaker unit to and from the connected, disconnected and test positions.

76. DC overcurrent relay is a device which functions when the cur rent in a DC circuit exceeds a given value.

77. Pulse transmitter is used to generate and transmit pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving device.

78. Phase angle measuring, or out-of-step protective relay is a device which functions at a predetermined phase angle between two voltages or between two currents or between voltage and current.

79. AC reclosing relay is a device which controls the automatic reclosing and locking out of an AC circuit interrupter.

80. Reserved for future application.

81. Frequency relay is a device which functions on a predetermined value of frequency either under or over or on normal system frequency or rate or change of frequency.

82. DC reclosing relay is a device which controls the automatic closing and reclosing of a DC circuit interruptor generally in response to load circuit conditions.

83. Automatic selective control, or transfer, relay is a device which operates to select automatically between certain sources or conditions in an equipment or performs a transfer operation automatically.

84. Operating mechanism is the complete electrical mechanism, or servo mechanism including the operating motor, solenoids position switches, etc., for a tap changer induction regulator of any piece of apparatus, which has no device function number.

85. Carrier, or pilot-wire, receiver relay is a device which is operated or restrained by a signal used in connection with carrier-current or DC pilot-wire fault directionally relaying.

86. Locking-out relay is an electrically operated band or electrically reset device which functions to shut down and ... and equipment out of service on the occurrence of abnormal conditions.

87. Differential protective relay is a protective device which functions on a percentage or phase angle or other quantitative difference of two currents or of some other electrical quantities.

88. Auxiliary motor, or motor generator, is one used for operating auxiliary equipment such as pumps, blowers, exciters, rotating magnetic amplifiers, etc.

89. Line switch is used as a disconnecting or isolating switch in an AC or DC power circuit when this device is electrically operated or has electrical accessories such as an auxiliary switch, magnetic lock, etc.

90. Regulating device functions to regulate a quantity or quantities, such as voltage current, power, speed frequency temperature and load, at a certain value or between certain limits for machines, the lines or other apparatus.

91. Voltage directional relay is a device which operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given direction.

92. Voltage and power directional relay is a device which permits or causes the connection of two circuits when the voltage difference between them exceeds a given value in a predetermined direction and causes these two circuits to be disconnected from each other when the power flowing between them exceeds a given value in the opposite direction.

93. Field changing contactor functions to increase or decrease in one step the value of field excitation on a machine.

94. Tripping, or trip-free, relay is a device which functions to trip a circuit breaker contactor or equipment or to permit immediate trip ping by other devices, or to prevent immediate re-closure of a circuit interruptor, in case it should open automatically even though its closing circuit is maintained closed.

95 to 99. Used only for specific applications on individual installations where none of the assigned numbered functions from 1 to 94 is suitable.

A1 Suffix Letters

Suffix letters are used with device function numbers for various purposes. In order to prevent possible conflict, any suffix used singly, or any combination of letters, denotes only one word or meaning in an individual equipment. All other words should be written out in full each time they are used or use a clearly distinctive abbreviation. Furthermore, the meaning of each single suffix letter, or combination of letters, should be clearly designated in the legend on the drawings or publications applying to the equipment.

The following suffix letters generally form part of the device function designation and thus are written directly behind the device number, such as 23X, 90V or 52RT.

These letters denote separate auxiliary devices, such as:

X auxiliary relay Y auxiliary relay Z auxiliary relay R raising relay L lowering relay O opening relay C closing relay CS control switch CL 'a' auxiliary switch relay OP 'b' auxiliary switch relay U 'up' position switch relay D 'down' position switch relay PB push button

Note: In the control of a circuit breaker with so-called X_Y relay control scheme, the X relay is the device whose main contacts are used to energize the closing coil and the contacts of the Y relay provide the anti-pump feature for the circuit breaker.

These letters indicate the condition or electrical quantity to which the device responds, or the medium in which it is located, such as:

A air or amperes C current E electrolyte F frequency or flow L level or liquid P power or pressure PF power factor O oil S speed T temperature V voltage, volts or vacuum VAR reactive power W water or watts

These letters denote the location of the main device in the circuit, or the type of circuit in which the device is used or the type of circuit or apparatus with which it is associated when this is necessary, such as:

A alarm or auxiliary power AC alternating current AN anode

B battery or blower, or bus BK brake BP bypass BT bus tie C c ... or condenser, compensator or carrier current CA Cathode DC direct current E exciter F feeder or field or filament G generator or ground a H heater or housing L line M motor or metering N network or neutral a P pump R reactor or rectifier S synchronizing T transformer or test or thyratron TH transformer (high-voltage side) TL transformer (low-voltage side) TM telemeter U unit

a. Suffix 'N' is generally in preference to 'G' for devices connected in the secondary neutral or current transformers, or in the secondary or a current trans former whose primary winding is located in the neutral or a measure or power transformer, except in the case of transmission line receiving, where the suffix 'G' is more commonly used for those relays which operate on ground faults.

These letters denote parts of the main device, divided in the two following categories: all parts, except auxiliary contacts and limit switches as covered later.

Many of these do not form part of the device number and should be writ ten directly below the device number, such as 43/A.

BB bucking bar (for high-speed DC circuit breaker) BK brake C coil or condenser or capacitor CC closing coil HC holding coil OS inductive shunt L lower operating coil M operating motor MF fly-ball motor ML load-limit motor MS speed adjusting or synchronizing motor S solenoid TC trip coil U upper operating coil V valve

All auxiliary contacts and limit switches for such devices and equipments as circuit breakers, contactors, valves and rheostats. These are designated as follows:

a. Auxiliary switch, open when the main device is in the de-energized or non-operated position.

b. Auxiliary switch, closed when the main device is in the de-energized or non-operated position.

aa. Auxiliary switch, open when the operating mechanism of the main device is in the de-energized or non-operated position.

bb. Auxiliary switch, closed when the operating mechanism of the main device is in the de-energized or non-operated position.

e, f, h, etc., ab, ad, etc., or ba, bc, bd, etc.

Special auxiliary switches other than a, b, aa and bb. Lower-case (small) letters are to be used for the above auxiliary switches.

Note: If several similar auxiliary switches are present on the same device they should be designated numerically, 1, 2, 3, etc., when necessary.

LC Latch-checking switch, closed when the circuit breaker mechanism linkage is re-latched after an opening operation of the circuit breaker.

LS limit switch.

These letters cover all other distinguishing features or characteristics or conditions, not previously described, which serve to describe the use of the device or its contacts in the equipment such as:

A accelerating or automatic B blocking or backup C close or cold D decelerating detonate or down E emergency F failure or forward H hot or high HR hand reset HS high speed IT inverse time L left or local or low or lower or leading M manual OFF, off, ON, on, O, open, P, polarizing, R, right, or raise, or reclosing, or receiving, or remote, or reverse s sending or swing T test or trip, or trailing TDC time-delay closing TDO time-delay opening U up

A2 Suffix Numbers

If two or more devices with the same function number and suffix letter (if used) are present in the same equipment they may be distinguished by numbered suffixes as for example 52X-1, 52X-2 and 52X-3, when necessary.

A3 Devices Performing More Than One Function

If one device performs two relatively important functions in an equipment so that it is desirable to identify both of these functions this may be done by using a double function number and name such as 27-59 undervoltage and overvoltage relay.


B1 General Circuit Elements

B2 Operating Mechanisms

B3 Switchgear

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