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
R.C.D. and Earth Electrode Calculations
- 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:
Sizing the C.P.C.
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.