Dc Through Ac Mcb's

[binky]

oh my god, not 3mm longer!!!!  :slap

We've got pfc of approaching 10k on 6k MCB's. Old install in metal containment. The decision was made to replace all effected with 10k variety.

Site engineers argued it's not been a problem in 40 odd years. Counter argument - yes it's 40
odd years old, time to expect issues.

 

pay attention, your 6ka MCBs will never see 10ka, perhaps that's why they haven't melted on 40 years nor did the re-wirable fuse fitted before that rated at 1.5ka.... :slap

 

[Sidewinder]

Not always true binky, I've measured PFC's at sub boards in excess of 15kA.

PFC @ origin only limited by DNO Tx impedance to 35kA.

Boards < 6m from the main switch gear.

 

[HandySparks]

oh my god, not 3mm longer!!!!  :slap

No mate. 4m longer.  :slap

 

[pickleye]

Irrelevant what fault current they will see in the real world.

The insurance company see unsatisfactory by the testers. So insurance is a problem until resolved.

If any one wants to sign off on leaving them in place, accepting all liability, I'm sure my boss will bung you a few beer tokens.

 

[binky]

No mate. 4m longer.  :slap

my bad :innocent

your real world example assumes a perfect dead short, which is highly unlikley, even if cutting a live cable. . Throw in  a nominal resistance of say 0.1ohm and it makes a huge difference

 

[binky]

Not always true binky, I've measured PFC's at sub boards in excess of 15kA.

PFC @ origin only limited by DNO Tx impedance to 35kA.

Boards < 6m from the main switch gear.

I you are getting those sort of figures, then I would argue the sub-main OCPD needs suitable rating.

 

[Marvo]

What type of panel is this? Is it a DB or MCC? Is there any manufacturers paperwork for the panel or can paperwork be obtained? It's possible the cascaded coordination of protective devices allows the use of OCPD's with what looks like inadequate KA ratings for the PFC. There may also be zones that were designed as 'fault free' that could also allow this if certain criteria was met.

As for battery PFC your standard PFC or impedance testers won't help you, there's calc method outlined in IEC61660 which sdhould be acceptable.

TBH this isn't the kind of design or remedy you should be responsible for, if you've got an engineer involved then you've done what you should.

 

[Sidewinder]

...

if you've got an engineer involved

....

Marvo, you hare having a laugh, you forget, in SA, an Engineer is a REAL Engineer.

In the UK it's a joke, the OP is at the sh*ty end of the stick, and is being landed with the pieces.

I can guarantee that no qualified Engineer competent to assess this job has been involved, and I bet also that no one that has been involved is competent to do the assessment.

This is why the OP is posting.

The nearest that he has managed to get to competent and qualified advice has come from here.

The poor sod, is but cannon fodder, and he is trying to avoid being blown up by the incoming canon fire!

Some of us on here, are actually registered and would be recognised as Engineers in Countries where the title is protected in law, this is not something that the people who should be supporting the OP are I suspect, else the poor sod would not be seeking advice here.

The job would have been right.

He's just "piggy in the middle", and will be used as the scapegoat when it goes wrong if the muppets can do it.

The poor sod is just trying his best, and being conscientious, and has NO backup from his so called employer it seems.

 

[binky]

Out of interest, what is the highest KA rating you can get on an MCB?

 

[Sidewinder]

10

 

[binky]

so what would cope with 15kA or higher? I'm assuming fuses??

 

[Marvo]

Gewiss have the MT250 25kA range which are s/pole and 3/pole but I'm not sure if they do oddities like SP+N etc. http://www.talermeister.de/haustechnik/installationstechnik/gewiss/mt250-ls-schalter-c20a-3p-25ka-3te/a-2081825/

Sorry I don't have an english link but I'm sure you could track them down locally.

 

[apprentice87]

So far as i know, and i am not sure about this mind, it is ok to have an MCB with a breaking capacity LESS than the PFC at that point so long as upstream of it you have a BS88 or whatever that HAS got sufficient breaking capacity. It is in the regs somewhere as far as i can remember.. I would have to go and look it up

john..

I used this MCCB on my shed electrics, it breaks plenty!!

http://s1178.photobucket.com/user/resistance87/media/MB235.jpg.html

john..

 

[Rob_the_rich]

So far as i know, and i am not sure about this mind, it is ok to have an MCB with a breaking capacity LESS than the PFC at that point so long as upstream of it you have a BS88 or whatever that HAS got sufficient breaking capacity. It is in the regs somewhere as far as i can remember.. I would have to go and look it up

john..

536.1

 

[Andy™]

so what would cope with 15kA or higher? I'm assuming fuses??

MCCB's & fuses

 

[Sidewinder]

That's fine however, after every trip you then must replace the MCB, as it has operated beyond it's design capacity and may be damaged, such that the next time it will not operate, unless, the upstream device also goes, and, you can prove that the energy let through of the upstream device is sufficiently limited to ensure that the lower rated device cannot be damaged.

Also, the DB that the lower rated device is installed within, must, be a type tested assembly, guaranteed by the OEM to be adequate for the fault current at the point of disconnection, i.e. the breaker.

 

[Sidewinder]

Also, don’t forget that Ics can be as low as half Icn for a 60947 device, so for a 15kA Icn rating, the Ics could be as low as 7k5.

Any PFC not otherwise limited by upstream devices above the Ics value renders the device replaceable at every trip, as above.Also, don’t forget that Ics can be as low as half Icn for a 60947 device, so for a 15kA Icn rating, the Ics could be as low as 7k5.

Any PFC not otherwise limited by upstream devices above the Ics value renders the device replaceable at every trip, as above.

 

[Marvo]

Marvo, you hare having a laugh, you forget, in SA, an Engineer is a REAL Engineer.

In the UK it's a joke, the OP is at the sh*ty end of the stick, and is being landed with the pieces.

I can guarantee that no qualified Engineer competent to assess this job has been involved............

Isn't Engineer a protected title in the UK? i was meaning a 'real' engineer and I didn't realise there was scope for ambiguity with the term....guess every day's a school day

 

[Sidewinder]

Isn't Engineer a protected title in the UK? i was meaning a 'real' engineer and I didn't realise there was scope for ambiguity with the term....guess every day's a school day

Is it F**k!

Anyone can call themselves an Engineer and get away with it.

We have Engineers who don't even do the work of a trade let alone a real Engineer.

Don't even get me on that soap box!!! ; (

 

[Essex1]

Who needs engineer when you have Amtech???