5.3.4 - Earth-fault loop impedance values

The over-riding requirement is that sufficient fault current must flow in the event of an earth fault to ensure that the protective device cuts off the supply before dangerous shock can occur. For normal 240 V systems, there are two levels of maximum disconnection time. These are:

For socket outlet circuits where equipment could be tightly grasped: 0.4 s

For fixed equipment where contact is unlikely to be so good: 5 s

The maximum disconnection time of 5 s also applies to feeders and sub-mains.

It must be appreciated that the longest disconnection times for protective devices, leading to the longest shock times and the greatest danger, will be associated with the lowest levels of fault current, and not, as is commonly believed, the highest levels.

Where the voltage is other than 240 V, [Table 41A] gives a range of disconnection times for socket outlet circuits, of which the lowest is 0.1 s for voltages exceeding 400 V.

In general, the requirement is that if a fault of negligible impedance occurs between a phase and earth, the earth-fault loop impedance must not be greater than the value calculated from..

	Zs <	Uo
		Ia
where	Zs =	the earth fault loop impedance (Ohms)
	Uo =	the system voltage to earth(V)
	Ia  =	the current causing automatic disconnection
	(operation of the protective device) in the required time [A]).

The earth fault loop values shown in [Tables 5.1, 5.2 and 5.4] depend on the supply voltage and assume, as shown in the Tables, a value of 240 V. Whilst it would appear that 240 V is likely to be the value of the supply voltage in Great Britain for the foreseeable future, it is not impossible that different values may apply. In such a case, the tabulated value for earth fault loop impedance should be modified using the formula:-

	Zs =	Zt x	U
			U240
where	Zs =	is the earth fault loop impedance required for safety
	Zt =	is the tabulated value of earth fault loop impedance
	U =	is the actual supply voltage
	U240 =	is the supply voltage assumed in the Table.

As an alternative to this calculation, a whole series of maximum values of earth fault loop impedance is given in {Table 5.1} (from [Table 41B]) for disconnection within 0.4 s. The reader should not think that these values are produced in some mysterious way - all are easily verified using the characteristic curves {Figs 3.13 to 3.19}.

For example, consider a 20 A HRC fuse to BS88 used in a 240 V system. The fuse characteristic is shown in {Fig 3.15}, and indicates that disconnection in 0.4 s requires a current of about 130 A. It is difficult (if not impossible) to be precise about this value of current, because it is between the 100 A and 150 A current graduations.

Using these values,
Zs =	Uo	=	240	Ohms = 1.84 Ohms
	Ia		130

Reference to {Table 5.1} shows that the stated value is 1.8 Oh,s, the discrepancy being due to the difficulty in reading the current with accuracy. {Tables 5.1 and 5.2} give maximum earth-fault loop impedance values for fuses and for miniature circuit breakers to give a minimum disconnection time of 0.4 s in the event of a zero impedance fault from phase to earth.

The reason for the inclusion of fixed equipment as well as distribution circuits in {Table 5.2} will become apparent later in this sub-section.

Table 5.1 - Maximum earth-fault loop impedance for 240 V socket ---------------- outlet circuits protected by fuses
Fuse rating (A)	Maximum earth-fault loop impedance (Ohms)
-	Cartridge BS 88	Cartridge BS 1361	Semi-enclosed BS3036
5	-	10.9	10.0
6	8.89	-	-
10	5.33	-	-
15	-	3.43	2.67
20	1.85	1.78	1.85
30	-	1.20	1.14
32	1.09	-	-
40	0.86	-	-
45	-	0.60	0.62

Table 5.2 - Maximum earth-fault loop impedance for 240 V circuits -----------------protected by miniature circuit breakers to give compliance ---------------- with 0.4 s disconnection time
-	Maximum earth-fault loop impedance (Ohms)
Device rating (A)	MCB type 1	MCB type 2	MCB type 3 and type C	MCB type B	MCB type D
5	12.00	6.86	4.80	-	2.40
6	10.00	5.71	4.00	8.00	2.00
10	6.00	3.43	2.40	4.80	1.20
15	4.00	2.29	1.60	-	0.80
16	3.75	2.14	1.50	3.00	0.75
20	3.00	1.71	1.20	2.40	0.60
25	2.40	1.37	0.96	1.92	0.48
30	2.00	1.14	0.80	-	0.40
32	1.88	1.07	0.75	1.50	0.38
40	1.5	0.86	0.60	1.20	0.30

The severity of the electric shock received when there is a phase to earth fault (indirect contact) depends entirely on the impedance of the circuit protective conductor. We saw in {3.4.3} and {Fig 3.8} how the volt drop across the protective conductor is applied to the person receiving the shock. Since this volt drop is equal to fault current times protective conductor impedance, if the protective conductor has a lower impedance the shock voltage will he less. Thus it can be sustained for a longer period without extreme danger.

Socket outlet circuits can therefore have a disconnection time of up to 5 s provided that the circuit protective conductor impedance's are no higher than shown in {Table 5.3} for various types of protection.

The reasoning behind this set of requirements becomes clearer if we take an example. {Table 5.3} shows that a 40 A cartridge fuse to BS 88 must have an associated protective conductor impedance of no more than 0.29 Ohms if it is to comply. Now look at the time/current characteristic for the fuse {Fig 3.15} from which we can see that the current for operation in 5 s is about 170 A. The maximum volt drop across the conductor (the shock voltage) is thus 170 x 0.29 or 49.3 V.

Table 5.3 - Maximum impedance of circuit protective conductors to ---------------- allow 5 s disconnection time for socket outlets
-	Maximum impedance of circuit protective conductor
Rating (A)	Fuse BS 88	Fuse BS 1361	Fuse BS 3036	MCB type 1	MCB type 2	MCB type 3 & C	MCB type B	MCB type D
5	-	3.25	3.25	2.50	1.43	1.00	-	0.50
6	2.48	-	-	2.08	1.19	0.83	1.67	0.42
10	1.48	-	-	1.25	0.71	0.50	1.00	0.25
15	-	0.96	0.96	0.83	0.48	0.33	-	-
16	0.83	-	-	0.78	0.45	0.31	0.63	0.16
20	0.55	0.55	0.63	0.63	0.36	0.25	0.50	0.12
25	0.43	-	-	-	-	-	-	0.10
30	-	0.36	0.43	0.42	0.24	0.17	-	-
32	0.34	-	-	0.39	0.22	0.16	0.31	0.08
40	0.26	-	-	0.31	0.18	0.13	0.25	0.06
45	-	0.18	0.24	0.28	0.16	0.11	0.22	0.06

 

Table 5.4 - Maximum earth-fault loop impedance for 240 V fixed ---------------- equipment distribution circuits protected by fuses
-	Maximum earth-fault loop impedance
Device rating (A)	Cartridge BS 88	Cartridge BS 1361	Semi-enclosed BS 3036
5	-	17.1	-
6	14.1	-	-
10	7.74	-	-
15	-	5.22	5.58
16	4.36	-	-
20	3.04	2.93	4.00
30	-	1.92	2.76
32	1.92	-	-
40	1.41	-	-
45	-	1.00	1.66
50	1.09	-	-

Application of the same reasoning to all the figures gives shock voltages of less than 50 V. This limitation on the impedance of the CPC is of particular importance in TT systems where it is likely that the resistance of the earth electrode to the general mass of earth will be high.

The breaking time of 5 s also applies to fixed equipment, so the earth-fault loop impedance values can be higher for these circuits, as well as for distribution circuits. For fuses, the maximum values of earth-fault loop impedance for fixed equipment are given in {Table 5.4}.

No separate values are given for miniature circuit breakers. Examination of the time/current characteristics {Figs 3.16 to 3.19} will reveal that there is no change at all in the current causing operation between 0.4 s and 5 s in all cases except the Type 1. Here, the vertical characteristic breaks off at 4 s, but this makes little difference to the protection. In this case, the values given in {Table 5.2} can be used for fixed equipment as well as for socket outlet circuits. An alternative is to calculate the loop impedance as described above.