pscc of breakers

[JohnW2]

IIRC there is a reg that lets you assume for most single phase supplies that the service fuse is providing back-up protection and so you don't need to worry about the breaking capacities of the MCBs

I can't say I can recall such a reg, but that doesn't mean much However, although such backup might suffice in a disconnection sense, I don't think it would make much sense in terms of the health of the MCB, which would very probably attempt to break the current (even if higher that it's rated max breaking current) before the fuse operated.

Kind Regards, John.

 

[ban-all-sheds]

Breaking Capacity.

Breaking capacity is the magnitude of fault current that the
device is able to disconnect safely. The rating for the device is
obtained by carrying out a series of tests. The devices are given
two separate ratings; a maximum and a service rating. Symbols
used to show this are...

Icn Rated short-circuit breaking capacity (BS EN 60898)

Icu Ultimate short-circuit capacity (BS EN 60947-2)

These symbols correspond to the maximum fault
current capable of being handled by a circuit breaker
and represents the maximum short-circuit current
which the breaker can interrupt and is to be compared
with the prospective fault current at the point of
installation.

Ics Service short-circuit breaking capacity
This is the magnitude of fault current at which the device
can be safely put back into service and represents the
maximum level of fault current operation after which further
service is resumed without loss of performance.

 

[JohnW2]

<pasted material from Hager>

Does this mean that you are agreeing with me? It seems to me that Icn must be at least as high as the PFC unless one can absolutely guarantee (and I can't see how one ever could) that some other device will disconnect the circuit before the MCB attempts to break it.

Kind Regards, John.

 

[ricicle]

Iirc if the consumer unit is EN 60439 and the breakers are to 60898/61009 with a 1361 main fuse then it doesn't matter what the breakers are rated at. The whole assembly is rated to 16kA. (which incidentally is the max. quoted DNO PSC figure anyway)

 

[Adam_151]

Iirc if the consumer unit is EN 60439 and the breakers are to 60898/61009 with a 1361 main fuse then it doesn't matter what the breakers are rated at. The whole assembly is rated to 16kA. (which incidentally is the max. quoted DNO PSC figure anyway)

Thats true, but thing to remember is that if you mix and match your breakers its no longer a type tested assembly to BSEN 60439 and you can't realy on annex ZA.

It applies to older equipment as well as 60493 con units as well, even the old wylex boards were allowed upto 16ka by BS5486:13

I've never seen anything even approaching 6kA on a domestic! Its also worth baring in mind that once you get down to that low level of supply impedance, the average loop tester isn't particualy accurate. Even the likes of the megger LTW425 with the high resolution setting struggle a bit, simply put because at this level the reading can be affected a lot by just how hard you push the probes into the copper

 

[ricicle]

And even if you were witnessing a domestic PSC reading of anything above about 10kA then meter inaccuracy, transformer impedance as well as increased copper resistance due to the heat generated by a fault would soon bring that fault current back down.

 

[JohnW2]

Iirc if the consumer unit is EN 60439 and the breakers are to 60898/61009 with a 1361 main fuse then it doesn't matter what the breakers are rated at. The whole assembly is rated to 16kA. (which incidentally is the max. quoted DNO PSC figure anyway)

If that's true, it brings me back to the point/question I keep raising. If the MCBs etc. have a 'maximum current breaking capacity' of, say, 6kA, does that not mean that their contacts could be damaged (might even weld together) if they attempted to break a current appreciably higher than that rating? If so, unless one could guarantee that that the main fuse would operate before the MCB attempted to break the current (and it seems very unlikely that one could guarantee that), would not the MCBs be 'at risk' even if the set-up were compliant (i.e. the installation as a whole would disconnect as required under fault conditions)?

.... and I remain interested to hear what sort of 'high' PFCs people do actually see on domestic installations. In my very limited experience, I haven't see or heard of any which are a lot more that 1kA, let alone 6kA or 10kA.

Kind Regards, John.

 

[EFLImpudence]

.
Just as a personal preference I would like the Zs at a socket outlet to be 0.038&#937;.

Can someone please explain how I may achieve this?

 

[Adam_151]

If that's true, it brings me back to the point/question I keep raising. If the MCBs etc. have a 'maximum current breaking capacity' of, say, 6kA, does that not mean that their contacts could be damaged (might even weld together) if they attempted to break a current appreciably higher than that rating?

Quite possibily, such a fault would have to occur quite near to the DB, something which the standards writers take as being unlikely, in which case a few metres of cable might also be damaged and in need of replacement, though chances are that if you nailed a cable half a meter from the con unit, you'd be replaceing from the con unit up to a joint box anyway... I assume the fact that the MCB might need replacement isn't a big worry...as long as it fails in a safe way then I assume its deemed good enough for an unlikely occurance. MCBs do get damaged when clearing faults sometimes, usually when someone repeatedly closes it on to a fault until it gives up the ghost

If so, unless one could guarantee that that the main fuse would operate before the MCB attempted to break the current (and it seems very unlikely that one could guarantee that)

The MCB will start to open immediatly upon detecting the fault, and may take typically a few tens of ms to fully open, a fuselink when faced with a fault of a few KA will be clear long below this time (fuses are constant let through energy devices, whereas with breakers its opertaing time thats constant). But what you have to remember is that the impedance of the supply network will limit the rise time of the fault, and the fuse will have cleared before the fault has risen to its theoretical maximum.

.... and I remain interested to hear what sort of 'high' PFCs people do actually see on domestic installations. In my very limited experience, I haven't see or heard of any which are a lot more that 1kA, let alone 6kA or 10kA.

If I see one in excess of about 2 and a half ka then thats the exception. Its about 1.7ka at the con unit at home and thats about the top end of normal for a PME domestic supply I find.

Even in the main switch rooms of sites with an on site transformer the other side of the wall the meter doesn't often report more than abou 7ka on any phase to neutral, doubled to 14ka for a fault involving multiple phases. This is a bit out though, the calaculations suggest around double that on the transformer terminals, then obviously the transformer tails will attenuate this a bit. But as I said, loop testers do struggle with low impedances like this as most of the impedance is reactive, and contact reistance is highly significant. Its not usually a problem that you cannot get a perfect measurment though. Most installations of this nature use BS88-2 fuses at this end of the installation, which are rated to break 80ka, and where breakers are used, the MCCBs usually are typically rated somewhere in the regien of 35ka

 

[ricicle]

.
Just as a personal preference I would like the Zs at a socket outlet to be 0.038&#937;.

Can someone please explain how I may achieve this?

Move the socket closer to the transformer.

 

[JohnW2]

Quite possibily, such a fault would have to occur quite near to the DB, something which the standards writers take as being unlikely .... I assume the fact that the MCB might need replacement isn't a big worry...as long as it fails in a safe way then I assume its deemed good enough for an unlikely occurance.

Fair enough. I suppose that is not that unreasonable an approach. As far as “as long as it fails in a safe way”, whilst that may true of the installation as a whole, I don’t think it can be guaranteed for the MCB itself, can it? – welded contacts would presumably not count as ‘failing in a safe way’.

The MCB will start to open immediatly upon detecting the fault, and may take typically a few tens of ms to fully open, a fuselink when faced with a fault of a few KA will be clear long below this time ....

Is not the problem there that, as you say, an MCB will start opening (or trying to open) immediately, and I would have thought that it is then (rather than when it is fully open) that problems (of contact arcing, maybe welding) are most likely to arise? However, if (as above) one is not too worried about the MCB being damaged, then I guess that’s not too much of an issue.

If I see one in excess of about 2 and a half ka then thats the exception. Its about 1.7ka at the con unit at home and thats about the top end of normal for a PME domestic supply I find.

Thanks. That’s roughly what I would have expected, and corresponds with my very limited experience.

Kind Regards, John.

 

[scousespark]

It's a single phase install, and we've had no confirmation of what current is being supplied, or confirmation of anything for that matter.
I might measure in adjacent building, isn't ideal though.
And thanks for that scousespark, but I'm pretty highly qualified, and this is just a fairly unusual situation I thought some helpful individual here may have been in before.

The original question shows you are out of your depth. You say you are pretty highly qualified, yet your posts indicate otherwise (or rather you lack the experience required for this job).

There is nothing unusual about this job.

 

[ban-all-sheds]

Does this mean that you are agreeing with me? It seems to me that Icn must be at least as high as the PFC unless one can absolutely guarantee (and I can't see how one ever could) that some other device will disconnect the circuit before the MCB attempts to break it.

The point I was trying to make is that beyond 6kA devices, Ics < Icn.

 

[JohnW2]

Does this mean that you are agreeing with me? It seems to me that Icn must be at least as high as the PFC unless one can absolutely guarantee (and I can't see how one ever could) that some other device will disconnect the circuit before the MCB attempts to break it.

The point I was trying to make is that beyond 6kA devices, Ics < Icn.

True, but that doesn't alter my statement that Icn must be at least as high as the PFC, does it? I'm also not yet convinced that one will ever get a PFC anything like as high as even 6kA, let alone above that, in a domestic installation

Kind Regards, John.

 

[RF Lighting]

Domestic install I did a couple of years ago.

At the incommer terminals: