Page 258 - Handbook of Electrical Engineering

242 HANDBOOK OF ELECTRICAL ENGINEERING

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If the 50 mm cable and a MCCB rating of 125 amps are chosen then the circuit earth loop
impedance is still too high by a reduced ratio of about 1.19:1. Hence an earth leakage circuit breaker
is still required.

Note: In most practical power systems of the TN or TT types it is found that an earth leak-
age core-balance relay is recommended for all LV motors above approximately 18.5 to
30 kW.
Note: Some oil companies specify a lower disconnection time t dis than 5.0 seconds, e.g., 1.0 second.
This significantly increases the disconnection current by a factor of about 3.0 times. This
ensures a much lower permissible limit to Z loopf , and thereby making it more necessary to
use an earth leakage circuit breaker. Indirectly this reduction in time should be accompanied
by ensuring that the earth return impedance Z mr (and Z er ) is kept very low i.e. as far below
0.1 ohms as possible. For an offshore platform this should be reasonably easy to achieve, e.g.
0.01 ohms, for a TT system because the general mass of steel is connected in parallel with
the neutral conductor if a 4-wire supply is provided. Even for a 3-wire supply the steelwork
impedance should be very low.

9.4.4 Protection against Overloading Current

IEC60364 Part 4, section 433, applies to cables and consumer equipment that are protected against
overloading current by a fuse or relay device at the source of supply. This requirement should not
be confused with protection against short-circuit currents that are disconnected in a short period of
time. Overloading currents tend to cause the protective device to disconnect the circuit only after a
long period of time has passed e.g. tens of minutes, one hour. The standard defines three particular
currents, I n , I B and I 2 as follows,

I n is the nominal current if this is non-adjustable, or the setting current I ns if this is
adjustable, of the protective device.
I 2 is the operating current of the protective device.
I B is the design current of the circuit. This will often be the rated current of the cable
under the ambient and grouping conditions of the installation.
I Z is the rated current of the lowest capacity component in the series circuit, but
excluding the load of the consumer.
However, if a cable feeds a motor then the rated current I Z of the motor should
be I B .

The standard requires the following constraints to be fulfilled,

a) I B ≤ I n ≤ I z (9.12)

and
b) I z ≤ 1.45I z (9.13)

The circuit diagram showing these currents for a motor consumer is Figure 9.10.

In this situation the main concern is the near-to-asymptotic behaviour of the protective device.