TYPE B MCBs

[pantaiema]

Dear Expert
In "OFFICE" environment for ring socket outlets, which one is more appropriate/widele used. 32A type B MCBs or 32A type C MCBs.
I understand the advantage of using Type B is that because they operate more instant than type C in case of overload but could cause nuisance trip.
Also I undersatand that both are allowed according to wiring regulatin BS7671. But what if you could help me to explain which one is commonly used the electrical contractor, nowadays.

Also, if we have a series of fixed connected load "6 No. of 100W exract fans in the toilet". What is the best ways to wire the FCU for these extract fans:
1. Using 32A MCB ring circuit to sockets, OR
2. Using 20A MCB radial circuit to sockets, OR
3. Or is that allowed by regulation to used radial circuit connected to 6A MCBs, assuming type C or D MCBs to be used.


Thank you very much for your valuable advise.

Pantaiema

 

[Qedelec]

Type C MCB's require a maximum Zs half that of Type B's so before considering replacing an MCB to cure nuisance tripping, measure the R1 + R2 value

 

[Albert]

Type C MCB's require a maximum Zs half that of Type B's so before considering replacing an MCB to cure nuisance tripping, measure the R1 + R2 value

And the value of the MCB depends on the load, if you are talking about 6 fans of 100W, you might not need more than 10A MCB. Normally you will not have a problem with nuisance tripping.

You will not have a problem with overload if you choose the correct size cable, MCB (value) for the load.

Nothing wrong with using MCB type C but you have to test the Zs to find whether it is suitable (as mentioned by Qedelec).

 

[Spark123]

If you are overloading the circuit causing the circuit protective device to operate it is a better solution to get another ring main installed. Changing the circuit protective device to a type C will only eliminate tripping of the initial surge, not a prolonged overload when there are too many appliances plugged in.

 

[ban-all-sheds]

If you've got a TN-S supply, you'll never get to 0.4s with Type Cs.

But surely, pantaiema, the electrician you are using will know all this? If he doesn't then put him back on his horse, send him packing, and get one that does.

 

[plugwash]

If you've got a TN-S supply, you'll never get to 0.4s with Type Cs.

small corrections

if you've got a worse allowed case TN-S supply , you'll never get to 0.4s with 32A Type Cs (and for that matter you won't with 63A type Bs either which could be a problem for those wan'ting to install submains).

lukilly afaict most real life TN-S systems are considerablly better than the worst allowed case.

 

[ban-all-sheds]

I should have said that you'll never get to 0.4s with a Type C with an In that you'd expect to use on whatever size cable it is.

And I take your point about the actual value of Ze, but what if it gets worse and does reach the maximum?

 

[Adam_151]

actual value of Ze, but what if it gets worse and does reach the maximum?

I'd say its a bad idea for it to just scrape by (though I wouldn't worry about that too much if it was on an RCD)

by your same logic you could ask what if it gets really bad, a corroded clamp on the sheath leaves you with a Ze more typical of TT than TN-S...