High PFC.

[GBDamo]

The company is in the process of dropping a gable end which requires many circuits being cut back and then reinstated.

I advised we do an EICR, although not due and for info only, to get a baseline on the installation before we start ripping it to pieces.

It was approved and despite the age and some iffy spurs it's in remarkable condition.

One thing that stood out was at the main DB and two sub boards the the ZDBs were very low giving PFCs in the range of 3-6KA



All the boards are populated with Hager two module RCBO which have a breaking capacity of 4000A.

Those in DB1 will almost certainly be under rated.

The other two DBs though have an additional 10mm earth run back to DB1 alongside the 16mm T&E sub main. Removing this brings the ZDBs up to sensible levels.

The incomer is TP TN-S with only one phase in use.

Is it likely that originally the Ze was much higher and external DNO works have brought this down, hence the additional 10mm earth?



Obviously I have no access to any previous results.

 

[pc1966]

Off hand I don't know the CPC size for 16mm T&E but it might be under the 10mm or so needed for bonding on a TN-C-S supply, so that is one possible reason for the additional conductor.
[automerge]1596041442[/automerge]
Same for the adiabatic limit depending on the supply fuse, etc.
[automerge]1596041643[/automerge]
Oddly enough this is opposite to my current project where I am seeing around 5kA measured at our origin switched-fuse but expected around double that. Not sure I really trust my MFT in that region though as 0.01 or so of test cable impedance difference makes a massive difference to the resulting PFC/PSSC.

 

[GBDamo]

Cheers but its TN-S and all bonding is direct to the MET and confirmed.
There may be some unknown unknowns lurking but there is nowt obvious.
[automerge]1596042080[/automerge]
[automerge]1596041643[/automerge]
Oddly enough this is opposite to my current project where I am seeing around 5kA measured at our origin switched-fuse but expected around double that. Not sure I really trust my MFT in that region though as 0.01 or so of test cable impedance difference makes a massive difference to the resulting PFC/PSSC.
[/QUOTE]

This, I'm too doubting my MFT and will be confirming with a Megger tomorrow.

 

[Ian1981]

Consumer units complying to BSEN 61439-3 have a declared Ka rating of 16Ka and if it is a domestic installation then you are not even required to measure it.
See appendix 14 of bs7671.
Also see the requirements for back up protection being provided by the DNO’s cut out fuse.
As long as the let through energy of the DNO fuse is equal to or lower than the upstream devices Ka , then back up protection is afforded.
If I’m wrong about this part then I’m sure someone can correct me.
But like I said it’s not required in a domestic installation where the CU has a 16Ka rating.

 

[pc1966]

Getting too high a PFC would mean lower cable R than the MFT has assumed, which would be unusual unless you had 3rd party probes.

Getting too low a PFC on the other hand just needs a bit of dirt/tarnishing/lack of tight banana plug fit, etc.

Of course suspect calibration could account for errors in both directions! My MFT is only 6 months old and hardly used so I'm expecting it to be in-cal but can only do simple checks on it using the low ohm measurement and some calibration resistors.

 

[GBDamo]

Consumer units complying to BSEN 61439-3 have a declared Ka rating of 16Ka and if it is a domestic installation then you are not even required to measure it.
See appendix 14 of bs7671.
Also see the requirements for back up protection being provided by the DNO’s cut out fuse.
As long as the let through energy of the DNO fuse is equal to or lower than the upstream devices Ka , then back up protection is afforded.
But like I said it’s not required in a domestic installation where the CU has a 16Ka rating.

So, as long as the CU the RCBOs are housed in are matched then the Breaking Capacity becomes that of the housing not that of the RCBO?

This is not domestic.

I'll have a look at appendix 14.

Ta

Isn't a 1361 holder rated at 16.5 KA?

 

[Wilko]

Sadly I’ve seen low Ze due to my measurement error a few times. Parallel earth paths not removed, tester lead calibration review and adding a long lead into the test usually resolved it.
Out of interest, can you check PFC on the outgoing L terminal of an MCB?

 

[GBDamo]

Sadly I’ve seen low Ze due to my measurement error a few times. Parallel earth paths not removed, tester lead calibration review and adding a long lead into the test usually resolved it.
Out of interest, can you check PFC on the outgoing L terminal of an MCB?

Will be looking to do a few things tomorrow.

Use a new MFT and insert a length of single into the test lead, test and deduct then calculate the PFC rather than reading

I definitely removed all parallel paths at source. The 2 sub boards maybe not.

 

[Ian1981]

So, as long as the CU the RCBOs are housed in are matched then the Breaking Capacity becomes that of the housing not that of the RCBO?

This is not domestic.

I'll have a look at appendix 14.

Ta

Isn't a 1361 holder rated at 16.5 KA?

Have a read of this

 

[Julie.]

So, as long as the CU the RCBOs are housed in are matched then the Breaking Capacity becomes that of the housing not that of the RCBO?

This is not domestic.

I'll have a look at appendix 14.

Ta

Isn't a 1361 holder rated at 16.5 KA?

Unfortunately with 100A or 80A incoming fuses, the let-through from 6kA perspective is still going to be in the order of 5kA or more, so relying on the upstream fuses isn't likely to be possible here, but you would need to know the actual fault current and fuse size/type to really check.
[automerge]1596047695[/automerge]

Will be looking to do a few things tomorrow.

Use a new MFT and insert a length of single into the test lead, test and deduct then calculate the PFC rather than reading

I definitely removed all parallel paths at source. The 2 sub boards maybe not.

For fault levels, you shouldn't be removing the parallel paths.

To determine the maximum Zs or Zdb the parallel paths need to be removed, this gives the worse case for making sure the protection will operate with minimum current.

However for fault level confirmation of equipment then you need to find out what the maximum is, not the minimum for operation.

It would be no use removing all parallel paths and saying the equivalent is suitable for the fault level- say 3kA then put parallel paths back in and see the actual fault current now be beyond the capacity of the equipment installed.