Ipfc/Ipsc measurement at 2nd distribution board

[filo]

Hi,
My first post - hope someone can help.

I need to test the prospective fault current at a garage consumer unit. The unit is supplied from an RCBO in the main consumer unit. I intend to carry out the test at the incoming terminals of the garage consumer unit switch with everything switched off except the main switch and the RCBO on the main consumer unit.

My question is this:
Is this likely to damage the RCBO? If so how should I carry out the test - temporarily bypass the RCBO? or can the test be avoided by doing a calculation (I have the prospective fault current at the main board and the R1+R2 of the distribution circuit).

Any clues would be appreciated.

 

[Sintra]

You can test the pfc on a no trip setting or test the Zs at the cu on a no trip setting and calculate.

 

[filo]

Thanks - will try Ipfc test - think my meter is automatically 'no trip' although it hasn't always worked.

At the risk of sounding a bit dim - would I be right in saying that the prospective fault current is likely to be significantly lower than at the main consumer unit because the R1+R2 of the distribution circuit is of roughly the same order as the Ze?

 

[Sintra]

Yes the further you go from the origin the lower the pfc will get.

 

[ian.settle1]

Most multi-function meters now give the PSC after obtaining a Zs reading as the Zs is divided into 230 which is a preset value.

 

[pushrod]

If you don't have a no trip function on your tester you can still get a reading for the the Zs at the submain.

Isolate the rcbo at the CU which is supplying the submain
link out the line and earth at the submain
At the main CU, test with your earth loop impedance tester, black probe to the isolated side of your rcbo and red probe at incoming line to rcbo or main switch. Your reading will be the external loop impedance for the submain. Divide that into your voltage and you have your pefc for your submain.
Make sure you remove the link at the submain before energising your rcbo.