Is there a guide as to when to use different types (B, C, D ) of MCB's please.
MCB types and their uses
- Thread starter clarrie
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type B for most domestic wiring.
type C for some lighting circuits and motor circuits
type D only for heavy industrial applications and where the Zs are stupidly low.
Why do you ask? Are you aware of why there are 3 trip characteristics, and what the characteristics are?
type C for some lighting circuits and motor circuits
type D only for heavy industrial applications and where the Zs are stupidly low.
Why do you ask? Are you aware of why there are 3 trip characteristics, and what the characteristics are?
No I am not aware at all. I have just had some wiring done by a recognised company in a church. They did not finish the job but ran heavily over on costs. I have been having a look at the job and see they used D10s for the lighting and C32s for the ring mains, B16s for spurs.
I also noted they used firetuf cabling for rings, spurs and lighting (2.5mm 2+earth) except when they were wiring the emergency lights where they used 4+earth unmarked/uniedentifiable cable.
I am wondering whether I should be pleased or worried.
I also noted they used firetuf cabling for rings, spurs and lighting (2.5mm 2+earth) except when they were wiring the emergency lights where they used 4+earth unmarked/uniedentifiable cable.
I am wondering whether I should be pleased or worried.
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crafty1289 said:
I'd say a low Zs was sensible, not stupid!!
securespark said:
It depends how low, I think that anything too far smaller than 0.08 at the board might start to make our undersized CPCs fail the adiabatic on an MCB protected circuit, especially if you want to try and make 2.5/1 comply...
did this recognised company give a cert? and if so can you scan and post it so we can look it over.
D10s seems seriously over the top for lighting and C32s seems like a poor choice for rings (though both could be acceptable if Zs figures are low enough). It seems almost like they just threw in whatever they could find.
you also wan't to find out what that cable they used for the emergency lights was. Is there a BS number printed on it?
D10s seems seriously over the top for lighting and C32s seems like a poor choice for rings (though both could be acceptable if Zs figures are low enough). It seems almost like they just threw in whatever they could find.
you also wan't to find out what that cable they used for the emergency lights was. Is there a BS number printed on it?
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Adam_151 said:
Hmm. I suppose if the EFLI is very low, the psc would be sky-high, exceeding the 6K breaking capacity....
The maximum permitted Zs value for a 10A type D is 1.20 ohms, so providing the cable runs are not excessively long and the external EFLI value is low enough these MCBs are ok, but I would have thought it would be better to have type C MCBs fitted on the lighting circuits. The only time we fit type D MCBs is on special motor circuits and dedicated welder supplies.
Wouldn't have thought you could get 6kA rated type D breakers. I have only ever seen 10kA rated. (Although even this might not be a high enough rating, having just finished working on an installation with an incoming Ipsc of over 12kA and fitted with some 6kA breakers )
Rob
Wouldn't have thought you could get 6kA rated type D breakers. I have only ever seen 10kA rated. (Although even this might not be a high enough rating, having just finished working on an installation with an incoming Ipsc of over 12kA and fitted with some 6kA breakers )
Rob
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This is something I've never fully understood. How can a circuit with a mega -low Zs (and consequently sky-high psc) be protected by breakers with a lower breaking capacity?
back up protection.
for example in a domestic install you have a nice fat HRC fuse installed by the REC that would take out a current of 6 kiloamps in fairly short order and has a breaking capacity up to at least 15 kiloamps.
in large commercial installs (incomer over 125A) you presumablly have MCCBs or switchfuses providing backup protection to the MCBs that are used for final distribution.
for example in a domestic install you have a nice fat HRC fuse installed by the REC that would take out a current of 6 kiloamps in fairly short order and has a breaking capacity up to at least 15 kiloamps.
in large commercial installs (incomer over 125A) you presumablly have MCCBs or switchfuses providing backup protection to the MCBs that are used for final distribution.
The incomer on my job was a three phase supply, with 300A REC fuses. (think BS1361's but may have been BS88's) It was connected direct to a dist. board in the switchroom, which contained a mix of 6kA and 10kA MCBs. There was no other protection against the high Ipsc currents . It is now happily running on a panel board full of 25kA rated MCCBs.
I guess if there had been a short to earth the REC fuses would have blown (12kA won't take long to blow a 300A fuse) but as far as I understand it there is a risk that the under-rated MCB's can be destroyed or weld them selves in the ON position, which is not good.
Rob
. It is now happily running on a panel board full of 25kA rated MCCBs. I guess if there had been a short to earth the REC fuses would have blown (12kA won't take long to blow a 300A fuse) but as far as I understand it there is a risk that the under-rated MCB's can be destroyed or weld them selves in the ON position, which is not good.
Rob
securespark said:
I asked a very similar question here: http://www.theiet.org/Forums/forum/messageview.cfm?catid=205&threadid=8613&highlight_key=y might be worth a read through, though it does take a little getting your head around
plugwash said:
I am awaiting the cert. I have not paid yet and will not until I have it and all questions/queries have been satisfied. I can see no BS number on the emergency lighting cable, but will take a closer look tomorrow and post back. Thanks!
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