OK to swap B MCB for Type C or does my C load have to go on it's own breaker?

Msitekkie · Mar 7, 2022

I have a customer who want's me to fit a 3Kw Patio heaterto a garage. - I believe this requires a Type C breaker. There is limited space in the board, so wanted to get peoples views on whether it's OK to swap an existing B type (used for sockets) for the C type, or whether the C type load needs to be on it's own breaker. I guess really the question is whether it's OK for the garage sockets to be on the C breaker.

bill01803 · Mar 7, 2022

What is the Ze and the R1 +R2

Msitekkie · Mar 7, 2022

Sorry I don't know yet, I'm still weighing the job up at the quoting stage. Interesting that your question suggests that makes a difference to the answer...

bill01803 said:
What is the Ze and the R1 +R2

sarky spark · Mar 7, 2022

If its a heater then there’s no need for a type C.

Msitekkie · Mar 7, 2022

"The unit should be protected by a suitably rated isolator and fuse or type C MCB. An inrush current (initial current through the lamp) can be expected at switch on. This current can be as high as 12 times the normal operating current, lasting less than 20 milliseconds. This effect can also vary depending on temperature. In extreme cases, a type D breaker may be required."
NB do not be thrown off by the use of the word "lamp" here.

But we digress. Even if type C were not required, for future reference I would still like to understand the scenario/implications of using sockets (with normal loads) in conjunction with a C type breaker.

sarky spark said:
If its a heater then there’s no need for a type C.

pc1966 · Mar 7, 2022

Msitekkie said:
"The unit should be protected by a suitably rated isolator and fuse or type C MCB. An inrush current (initial current through the lamp) can be expected at switch on. This current can be as high as 12 times the normal operating current, lasting less than 20 milliseconds. This effect can also vary depending on temperature. In extreme cases, a type D breaker may be required."
NB do not be thrown off by the use of the word "lamp" here.

OK so some "halogen" style heater?

Msitekkie said:
But we digress. Even if type C were not required, for future reference I would still like to understand the scenario/implications of using sockets (with normal loads) in conjunction with a C type breaker.

The real difference between B/C/D is the "instant" trip point which results in quick disconnection for fault protection, rather than the thermal overload part. The regulations require automatic disconnection in under 0.4s for a nominal 230V TN supplied system so for MCB clearing of faults you need to reach that point.

So as you go B -> C -> D your maximum end of circuit Zs halves each time.

The UK style of 13A sockets don't care, they have their own fuse for fault/overload disconnection, but your circuit design for the socket fixed wiring becomes harder to meet for long runs. Yes, you can also meet disconnection on the RCD side of things quite legitimately (as you almost always need to on a TT supply) but I personally would aim to meet it on the MCB side if practical.

pc1966 · Mar 7, 2022

Just to add, that patio heater's inrush is specified at 157A (12 * 13.04A), which is pretty punishing for any switches. But it is implying you would need a 32A C-curve supply or 16A D-curve.

It also puts it in the middle of the standard 13A fuse's trip curve, so I suspect this might need a dedicated 16A supply with D-curve MCB and a 20A isolation switch (i.e. not a socket of FCU).

[ElectriciansForums.net] OK to swap B MCB for Type C or does my C load have to go on it's own breaker?

Msitekkie · Mar 7, 2022

pc1966 said:
OK so some "halogen" style heater?

The real difference between B/C/D is the "instant" trip point which results in quick disconnection for fault protection, rather than the thermal overload part. The regulations require automatic disconnection in under 0.4s for a nominal 230V TN supplied system so for MCB clearing of faults you need to reach that point.

So as you go B -> C -> D your maximum end of circuit Zs halves each time.

The UK style of 13A sockets don't care, they have their own fuse for fault/overload disconnection, but your circuit design for the socket fixed wiring becomes harder to meet for long runs. Yes, you can also meet disconnection on the RCD side of things quite legitimately (as you almost always need to on a TT supply) but I personally would aim to meet it on the MCB side if practical.

That's really helpful, thanks.