3/4 Rule of thumb Inspection and Testing. 2391 Exam

So what is this rule of thumb?

When carrying out testing of an electrical installation the value of the phase and circuit protective conductors resistance is measured together to give (R1+R2).
Ze which could have been either measured, calculated or obtained by enquiry is then added to (R1+R2) to obtain Zs, but you knew that.
For a particular protective device BS 7671 gives tables as to the maximum value of Zs that can achieve disconnection within a specified time.
The problem is that when the value of (R1+R2) is measured the temperature of the conductors are likely to be at ambient temperature. Trouble is that the tables in BS 7671 are not relevant to Zs values measured at ambient temperature.
The resistance of copper is proportional to the temperature, so if the temperature drops the resistance drops.
Rather than having to measure the ambient temperature the 'rule of thumb' method is allowed.

To compensate for the lower temperature the maximum value of the Zs allowed needs to be lowered. 3/4 of the value of Zs is the worst case and thus acceptable to use.


A Zs in the table of say 4 ohms. would result in a corrected value of 4 times 3/4 which is now 3 ohms.

From a C&G 2391 viewpoint you will be given a Zs maximum value for a circuit and a set of Zs values that were calculated or else given (R1+R2) to which you need to add Ze to.

Show the formula you are using so that even if your maths are wrong you will still gets marks awarded.
Corrected Value For Zs = ZsMax X 3/4.

Provided that the calculated value is less than the corrected value then the results are acceptable.