Protective conductor size... 2 contradicting regs?

DNS1 · Jan 13, 2012

Been reading up on the regs regarding protective conductors and doing a few calculations for imaginary circuits.

What is the point in having Table 54.7 in 543.1.4 if the values contradict those calculated using the adiabatic equation are much lower?

Is 54.7 just there as a time saver so you can just comply with that if you can't be bothered to do the calculations?

Please excuse my confusion, I've just spent hours with the books trying to work out how 2.5mm T+E with a 1.5mm cpc is compliant, as it contradicts the table!

baldsparkies · Jan 13, 2012

Are things really what they appear ??

Example of use of Adiabatic Equation.

Deleted member 26818 · Jan 13, 2012

Yes the table is there for people who can't be asked to conduct the calculation.
As indicated by the first line of 543.1.4.

DNS1 · Jan 13, 2012

Thanks guys!

ackbarthestar · Jan 13, 2012

The worst possible case, where a protective conductor provides earth fault protection for a number of circuits, such as an earthing conductor then you have a choice :
1/ Use the adiabatic formula 543.1.3
or
2/ By selection from table 543.1.4

I've always wondered why there was such a large difference between the calculated value and the tabulated value.
More guidance is given in 543.1.1 in that the minimum values allowable by calculation where the protective conductor is effectively separate from the live conductors, are 2.5mm2, protected and 4.0mm2 non-protected.

However, to cover all possible scenarios such as extra mechanical protection or/and non-competence of installation owners etc., table 54.7 is provided.

There is one partial exception where the protective conductor is the earthing conductor, usually associated with TT systems, providing protection for the whole installation and then reference to table 54.1 is made.

In other words, as mentioned earlier, if your fingers struggle using (√) the square route function on a calculator then use the table provided .

Engineer54 · Jan 13, 2012

There is no contradiction on protective conductor sizing, only choice!! Those sizes stated in 543.1.4 will cover your earthing requirements in all circumstances. While 543.1.3 will give you a minimum size to cover your earthing requirements in a specific circumstance!!

As others have stated, you have a choice of which Reg to use. The adiabatic formula is useful when you have an existing conductor size that maybe difficult to replace or upgrade. Using the formula, will tell you if the smaller CSA conductor will still comply ....or as it maybe, ...Not!!!

Even the adiabatic formula has a safety margin, but we won't go there. All you need to understand, is that you are given a choice of determining your protective conductor size...