Testing requirements from the regs

[simon williams]

Hi everyone, ive just started my apprenticeship and im looking through the regs trying to find the requirements for things like the maximum permitted value of r1+r2 values and loop tests, rcd breaking times for domestic etc. but cant find it in the regs or the on site guide. so if you can help that would be great.

simon

 

[industryspark]

hi

think you are referring to earth fault loop impedance.

equation for this is zs(efli) = ze+R1+R2

ze is the impedance external to the installation
R1 is the resistance of the phase conductor
R2 is the resistance of the cpc

when you have your zs value you can compare it to the regs tables to see if its an exceptable value for the type and rating of protective device in the circuit.

you will have to take 75% of your measured value to compare to the table in the regs. this 75% is a rule of thumb value 'allowing' you to skip the proper correction factors.

think the tables are on page 46, its 40 something definetly.

if you have further questions about this or anything else you dont understand this is a great site with some knowledgable sparks who you can ask.

 

[Spark123]

Hi everyone, ive just started my apprenticeship and im looking through the regs trying to find the requirements for things like the maximum permitted value of r1+r2 values and loop tests, rcd breaking times for domestic etc. but cant find it in the regs or the on site guide. so if you can help that would be great.

simon

You'll probably find this out in due course, r1+r2 is different to R1+R2. r1+r2 being the end-end resistances of the phase and earth conductors in a ring final socket circuit.

Your maximum Zs depends on the environment and your type of protection. When using circuit breakers the maximum efli is the same for both 0.4s and 5s, see appx 3 in BS7671. From these charts you will see that the current required to operate a type B MCB is 5x its rating hence for a 6A the current required is 30A. As the fault circuit involves all the cable from the distribution transformer, Uoc (the open circuit voltage at the supply transformer) is used which is nominally 240v. From ohms law, 240v/30A=8ohms. This refers to the cable at its maximum operating temperature, as a copper cable heats up its resistance increases. As when you are conducting an EFLI test the conductor should not be at its max temp, temperature correction is applied before comparing. For a copper cable measured at 20deg C whos maximum temp is 70deg C there is a 50deg C difference. The temperature coefficient for copper is 0.004 per degC hence for a drop of 50deg C the measured efli is required to be 0.004 x 50 = 20% lower (which is 80% of the original value). So for a cable protected by a 6A type B MCB which the maximum efli at 70deg C is 8ohms, its maximum measured value at 20deg C will be 6.4ohms. If you look at table 41B2 in bs7671 these are the maximum values, in table 2D of appx 2 in the OSG these are maximum measured values at 20deg C.

 

[simon williams]

that really helped guys thanks very much for your time.

simon