Siemens MCB advice please.

[pastedavid]

Hello,

(In the spirit of asking a stupid question rather than making a stupid mistake...)

I have a Siemens lecky board for the garage which uses 5SX21 circuit breakers. They are marked as Type B. The supplied BC are all 32 amp, but I need other ratings. The CBs I can see are Type C but they look physically identical.

Is there any reason the Type C CB can't be used along side the Type B?

Thanks in advance for any help.

David Paste.

[John Rumm]

There is no reason you can't fit a type C in the box, however they do have a different instantaneous trip characteristic, needing typically

10x the rated current to trip "instantly", rather than the normal 5x of a type B. This means that with the higher current rated devices you will need to check your maximum earth loop impedance on the circuit in question very carefully to make sure you maintain fault protection.

Have a look at:

for a C32 MCB, you could need 320A of fault current to ensure instant operation. That means a maximum loop impedance of 320/230 = 0.7 ohms. So you could not use one on a TN-S installation unless you actually measure Ze since you would otherwise need to assume Ze of 0.8 ohms at the origin, let alone the end of the circuit.

Smaller rated devices are obviously more forgiving. I routinely use 6A type C devices on lighting circuits since they are less likely to nuisance trip on filament bulb failures.

[pastedavid]

Thanks very much for the info - much appreciated!

[meow2222]

kers. They are marked as Type B. The supplied BC are all 32 amp, but I need= other ratings. The CBs I can see are Type C but they look physically ident= ical.

You should use type B unless there's a specific reason for Bs, as Bs offer = better protection. Lighting circuits migh be a fair use for type C, but whe= ther they actually do avoid tripping on lamp arcover is open to question. A= s mentioned you also need to meet more stringent circuit resistances with C= s.

NT