PFC

[thomo27]

Just a quick question in reference to Prospective Fault Current testing..

Three Phase Installation which give readings of 8.6KA between L1 and Earth , 8.9KA between L2 and Earth and 6.1KA between L3 and Earth..

Am I right in saying that the rule of thumb method i.e. x the reading by 2 applies to this test ??

 

[GaryMo]

I take it your meter isn't capable of testing PSCC at 415V?
If not then yes, PFC will be noted as twice the value of the highest measured PSCC which errs on the side of caution.

 

[thomo27]

Its a Megger 1552 so I believe not..

Ive since had a think and am I right in saying that its the PSCC that you double and not the PEFC ? Imthinking that the highest current would be between phases but as the 1552 cant test between phases you double it ?

 

[GaryMo]

Its a Megger 1552 so I believe not..

Ive since had a think and am I right in saying that its the PSCC that you double and not the PEFC ? Imthinking that the highest current would be between phases but as the 1552 cant test between phases you double it ?

My Megger 1552 measures PSCC between phases OK.
You're correct, it's the PSCC that's doubled (highest of L1-N, L2-N or L3-N), and not PEFC as I wrote.

 

[thomo27]

Its a Megger 1552 so I believe not..

Ive since had a think and am I right in saying that its the PSCC that you double and not the PEFC ? Imthinking that the highest current would be between phases but as the 1552 cant test between phases you double it ?

My Megger 1552 measures PSCC between phases OK.
You're correct, it's the PSCC that's doubled (highest of L1-N, L2-N or L3-N), and not PEFC as I wrote.

Oh OK,ive always used the company test kit and the guys have always just tested L1-N, L2-N and L3-N seperatly i.e red probe on L1 and Black probe to Earth..

Yes after having a think about I was struggling to see how I could double 8.9KA to get 17.8KA when the MCB's are rated at 10KA !!

 

[plugwash]

Be aware that if the neutral is bad it is possible for the phase-phase fault current to be more than twice the phase-neutral fault current.

 

[RF Lighting]

Its a Megger 1552 so I believe not..

Ive since had a think and am I right in saying that its the PSCC that you double and not the PEFC ? Imthinking that the highest current would be between phases but as the 1552 cant test between phases you double it ?

My Megger 1552 measures PSCC between phases OK.
You're correct, it's the PSCC that's doubled (highest of L1-N, L2-N or L3-N), and not PEFC as I wrote.

Oh OK,ive always used the company test kit and the guys have always just tested L1-N, L2-N and L3-N seperatly i.e red probe on L1 and Black probe to Earth..

Yes after having a think about I was struggling to see how I could double 8.9KA to get 17.8KA when the MCB's are rated at 10KA !!

Normally L-N will be higher than L-E

If your installation is backed by a 100A BS1361 DNO fuse then the MCBs have an assumed rating of 16kA

 

[Adam_151]

If your installation is backed by a 100A BS1361 DNO fuse then the MCBs have an assumed rating of 16kA

Not quite, the MCBs don't... the consumer unit as a type tested assembly has the 16ka rating. Which can cause issues if someone has mixed and matched breakers!

At the kind of built current the OP is measuring, BS88 fuselinks might be a better chocie than MCBs, and certainly above about 3ka PEFC one needs to be ensuring that they check the adiabatic for any reduced CSA CPCs...

Its worth noting that with a PEFC of 8.6ka thats an impedance of 0.0268, a standard loop tester will be unable to measure such a loop impedance with any degree of accuracy, and even the amount you did the probes into the busbar will have a large effect. And if your leads are fused in the tips (which at this fault level they bloody should be*) then the fuses can often add a bit... my fused leads read about 0.03 higher than my unfused ones


*The megger ones arn't fused, the fuse in the instrument, Megger state that on low impedance supplies its up to you to risk assess whether you need fused tips