Pfc and Ze - values by enquiry? (UK)

[PhilT74]

Hi folks,
When installing a new board in a domestic situation, you have the option of aquiring the Prospective fualt current, and earth loop impedance values by enquiry. Who do you contact to for these details?

 

[ScottyUK]

Usually the REC responsible for the distribution network serving the premises will be able to provide this information. The values will be typicals, and they will almost certainly be conservative which leads you toward over-engineering your installation. Can't you hire a loop tester and get actual values? Or is this a new-build?






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If we learn from our mistakes,
I'm getting a great education!

 

[panelman]

you need to measure pscc & earth loop on the various circuits after you have installed the board anyway so why not just measure them at the supply too

as scotty says, if you ask the LEB they will give you figures that are high and often by quite a lot. We recently asked for some figures and were told 16kA (P-P), when we measured it it was only 4kA (P-E so maybe 8kA P-P)

 

[PhilT74]

Looks like i'll just have to take a couple extra readings, I just thought it could save a bit of hassle disconnecting the earth bound to measure Ze.

 

[sparksski]

PhilT74

The REC always used to quote you 16KA (@ 0.55pf) as a maximum potential, this was assuming you were located next to thier transformer, as you found when you move away from the source and measured the reading it is significantly lower. Their figure is a distance of 35M for 4KA.

There are standard figures accepted and quoted for use typically 0.35 ohm for a TNCS - PME system, and 0.8 ohms for a TNS system as I recall. These can be used on your paperwork as the REC will quote these if you enquire. There is no substitute for a real test though.

Regards