Factors that affect insulation resistance

Insulation resistance 50 MΩ

There is no such thing as a perfect insulator and there will always be a very small amount of leakage current.

What would the effect on the I.R value be if the cable length was doubled?

Insulation resistance halved

If a circuit was added to an installation how would this affect the overall Insulation Resistance?

Overall Insulation Resistance would decrease, because it’s one more resistor in parallel

If a circuit was removed from an installation how would this affect the overall Insulation Resistance?

Overall Insulation Resistance would increase, because it’s one less resistor in parallel

RCD and Earth Electrode Calculations

The Facts:

• BS 7671 states that the max touch voltage should not exceed 50v.
• Where max values of Zs cannot be met due to a high earth loop impedance path (TT systems) the installation must be protected by an RCD of a rating necessary to keep touch voltage below max’ levels.
• Where a socket outlet supplies equipment used outside the Equipotential Zone supplementary protection against Direct Contact must be provided by a 30mA RCD.
• To comply with BS7671, most domestic circuits will be protected by a 30mA RCD.

The following calculations are just a case of knowing the above facts and being able to apply Ohm’s Law.

Where TT systems are used incorporating a 100mA rcd calculate the max theoretical value of the earth electrode (Ra) for:

A caravan site installation:  R = V/I = 50 / 0.1 = 500 ohms
These are theoretical values and BS 7671 states that a reading of above 200 ohms
would require further investigation.

The measured value of an earth electrode forming part of a TT system at the caravan site is 78 ohms. Determine the max permissible residual operating current of the RCD.
I = V/R = 50 / 78 = 640 mA

Sizing the CPC

The c.p.c is usually smaller than the Live conductors and will produce more heat, its size must be at least equal to:

• S is the minimum protective conductor c.s.a (mm²)
• I is the fault current (A)
• t is the opening time of the protective device (s)
• k is a factor depending on the conductor material, copper is 115.

This formula is used to prove that a reduced size cpc is capable of carrying high levels of fault current.