Page 358 - Handbook of Electrical Engineering

PROTECTIVE RELAY COORDINATION 345

12.8.4 Core Balance Earth Fault

This function is occasionally required because the earth loop impedance is too high. Most of the
impedance is in the armouring of the cable if the armouring is chosen to be braiding rather than
wires. If the route length is short then the problem may not arise, but for good design practice it is
not worth making exceptions for short routes. Core balance protection is normally required in these
circumstances for motor ratings above approximately 18.5 kW. A core balance current transformer
and a 50 N relay is used with a circuit breaker, or a 51 N relay with a contactor–fuse combination.
The sensitivity of the scheme should allow an earth fault current in the order of 30 mA to be
detected and reliably tripped.

12.8.5 Stalling Current

Low voltage motors used in the oil industry usually have modest starting times, since the majority of
their driven machines are pumps. Reciprocating compressors and ventilation fans can have reasonably
long starting times. It is therefore not normally necessary to provide special relays to detect the
stalling condition.

12.8.6 Limitation to the Number of Successive Starts

Low voltage motors are robust machines and can tolerate being restarted several times in succession.
It is not normal practice to provide special facilities to limit the number of starts in a predetermined
period of time. Modern motor control centres often have more sophisticated ‘motor management’
features than older equipment. It is reasonably easy to provide this requirement if the ‘motor man-
agement’ approach is adopted for the motor control centre.

12.9 LOW VOLTAGE STATIC LOAD PROTECTION

Static loads encompass heaters, battery chargers, uninterruptible power supplies, lighting distribution
boards, socket outlets, cathodic protection, navigational aids, computers, public address, radio com-
munication and the like. Excluded are loads that are not predominantly composed of motors. The
load may have fractional kW motors for cooling fans.
They are essentially constant current loads that have a power factor near or equal to unity.

The protection required is usually kept as simple as possible, consisting of,
• Time-delayed overcurrent.
• Instantaneous or high-set overcurrent.
• Core balance earth fault.

The circuit may be controlled by a circuit breaker or a combination of a contactor and fuses.
In some circuits that are controlled frequently as in the case of heaters controlled by thermostats
or thermometers, the main protection may be incorporated into a circuit breaker whilst the control
would be given by a contactor.