Page 544 - Handbook of Electrical Engineering

NUMBERING SYSTEM FOR PROTECTIVE, CONTROL AND INDICATION DEVICES 535

25. LV switchgear bus-section and bus-coupler circuit breakers.
86.
26. LV interconnectors, single and parallel cables, protection at both ends.
51, 51N, 86.
27. LV DOL induction motors, 37 kW to 250 kW. See Figure 12.19.
46, 49, 50, 50N see note 15, 86.
28. LV DOL induction motors, below 37 kW. See Figure 12.19.
49 see note 16, 50 see note 16.
29. LV static loads and feeders to distribution boards.
51, 51N see note 17, 86 see note 18.

C.1.1 Notes to sub-section C.1

1. In the situation where a generator has a unit transformer it is recommended that two forms for
differential protection are provided. One will be for the generator by itself, and denoted 87G. The
second will be an overall scheme for the generator and the unit transformer, and denoted 87T.
2. Bus-zone protection is recommended to be of the high-speed balanced voltage type. Each section
of busbars should be protected as one zone. There are other options available that are technically
more complex and hence more expensive. It is normal practice to overlap adjacent zones at bus-
section and bus-coupler circuit breakers. The bus-zone protection scheme can also be provided
with a test facility that can be used while the switchboard is in its normally energized state.
3. The undervoltage relay is usually provided with time delay settings, and used to trip the consumers
fed from the particular bus-section.
4. A device required for starting and stopping forced air-cooling fans.
5. An unrestricted earth fault relay that is connected in the star-point earth circuit of the equipment
being protected. The characteristic is time dependent so that time coordination is achieved with
the 50 N devices downstream.
6. A restricted earth fault relay is used with generators and transformers to detect internal faults.
The relay will usually be of the voltage operated instantaneous type.
7. The generator switchgear is recommended to have two separate lockout relays. One will receive
all the electrical protection relay trip signals, and denoted as 86–1. The other will receive a master
trip signal (or several trip signals) from the turbine unit control panel, and denoted as 86–2.
8. Normally only required on critical circuits.
9. Modern relays combine the 46, 49, 50, 51 N and 86 functions; together with others such as
motor stalling, number of starts, thermal state at the time of starting, undercurrent, overloading,
high-set current limit.
10. The manufacturers of the static loads and variable speed drives may require special protection
devices for their equipment.
11. The manufacturer of the variable speed drive may provide some protection devices in the unit
control panel. These devices may be inter-tripped with the main circuit breaker.
12. The manufacturer of the variable speed drive may recommend the 87 relay. It may be part of
the UCP as mentioned in Note 11. It should not be necessary for motors below 2.5 MW.