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## 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 (mm
^{2}) - 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.