High PFC issues

Where exactly are the existing MCBs and RCBOs and where exactly was the prospective fault current measured ?

You will probably find that most of the installation is fine as the fault current declines with distance from the transformer.

For those parts of the installation close to the transformer then providing back up protection by HRC fuses is one option. Note for example that domestic installations if close to the substation often have a fault current in excess of the breaking capacity of the installed MCBs or RCBOs, this is acceptable due to the back up protection provided by the DNO service fuse.

Hm, some problem there, Ze and PFC don't correspond very well. With the TX close, it is likely to be TN-S, but still, Ze is much higher than the PFC suggests. One point is that an MFT is not very accurate at low Ze or High PFC. Your Ze is a factor of 10 or so out compared to the PFC, and 11.7 kA 20 m from a 500 kVA TX could well be about right. If Ze was really 0.04 Ohms it would be more reasonable, but an MFT would be fairly inaccurate at that resistance. 11.7kA is sufficiently close to 10 kA for those to be OK unless you can get a fully accurate measurement, or a few metres of cable between the Main MCCB  or whatever and the next stage CPDs will get you to a figure which is within RCBO range. Are you sure you need RCD protection everywhere in commercial premises and is this the right place to put it if required?

Even if the Ze is a typo for 0.04 ohms, the figures still do not correspond.

AFAIK, testers measure EFLI. They then do the sums using Ohm's law. 230/0.04 = 5750. Assuming a 3-phase supply and PEFC ≈ PSSC, if you use the rule of thumb of doubling the single phase PSSC, that does give 11.5 kA.

If the installation is protected by BS88 or similar fuses, then they will give you protection against the higher fault currents.

If it is a panel board supplying the DB, then the breakers in that should give enough protection for 16k + amps.

seconded, unless there LN impedance and the L_E impedances are very different, suggesting another problem if it is TNCS.

0.4 ohms is more like 600A, not really enough to clear anything more than a 60A C type promptly

Or 11kA fault current would need a loop impedance of more like 0.02 ohms !!.

You could perhaps have an LN impedance of 0.02 and an LE of 0.4, but I;d expect them to be closer than that, and I'd be looking out for corroded bolts or loose lugs in the earth path.

But look at the BS88 let through energy - for a dead short close to the MCBs, the fuse may blow first, but it will almost certainly still protect the breakers from damage, even if they are 6kA types.

In addition real faults are very rarely zero impedance if they did, they would be silent, but all that flash crackle and pop takes real energy, and therefore a real resistance, so the fuses will not get popped that often.

Only the cable upstream of the fuse will see the full fault current.

Mike.

Tama: 
 

Hi all,

First post coming in…

I have a installation that we have been asked to Test, I conducted the usual tests for the Ze & Pfc.  I got 0.4 ohms & 11.7kA respectively.  Its a commercial block and the transformer is behind a locked door about 20m away, so probably reasonable to assume it’s accurate.  So here is my question given the absence of 15kA RCBO’s and the rarity 15kA MCB’s, what are my options?

Thanks in advance.

What type of consumer unit is it?

If it's a type-tested unit to BS EN 60439 or BS EN 61439, tested in accordance with the UK “national Annex” of one of those standards, and has all the manufacturer's recommended devices in it for the product range, then those standards permit protective devices with lower fault current ratings (usually 6 kA mcb's and RCBO's) to be used for prospective fault currents up to 16 kA - provided the service fuse is 100 A or less to either BS 1361 or BS 88-3. Where a specific protective device leads to a prospective fault current rating, this is termed in BS 7671 a “conditional prospective fault current rating”.

See:

• Regulation 536.4.201 (pages 179-180 of BS 7671:2018)
• Page 42 of Guidance Note 1 (8th Edition)
• Page 72 of Guidance Note 3 (8th Edition)

Thank you all for your help and advise. I think I have the answer I was looking for.

more info for those that want it.

• TNCS v TNS as far as I can ascertain it is a TNS, based upon the nature of the installation and what I have seen to date that is, however I can’t be certain as the header is a sealed unit and obviously I can’t break it to investigate.  The local authority aren’t much help in confirming that either.
• There is no Switch Panel, just the Standard DB with standard MCB’s / RCBO’s and a factory for MCCB as the main switch
• The DB is a Merlin Gerin Isobar 4c, there is a mix of Schneider & Merlin MCB’s fitted which is acceptable as they are effectively the same brand and susiquently tested to the same standards.

“they are effectively the same brand and susiquently tested to the same standards.”  in essence perhaps so but in practice will the manufacturer actually confirm that to CYA? perhaps not

ebee: 
 

“they are effectively the same brand and susiquently tested to the same standards.”  in essence perhaps so but in practice will the manufacturer actually confirm that to CYA? perhaps not

In this particular case it's not so much that it's “obvious that the parts are identical even though they've been sourced from different manufacturer” but that Schneider and Merlin Gerin are the same manufacturer (since Schneider bought out Merlin Gerin, and have proceeded to re-brand the existing Merlin range with SE logos).

  - Andy.