C type MCB safe for general garage use?

Mkol

Mkol

Member
Hi folks, i have a small consumer unit in my garage running 2 circuits one for lights and one for sockets. this is fed as a from the larger unit in the house on a 16amp MCB.
I have recently started to use a compressor that trips the mcb in the house when it starts, as a very temporary measure ive been getting round this by using the compressor on a extension cable which reduces the current enough to stop it tripping. (obviously not ideal in any way)

my question is can i replace the MCB in the house with a C type in order to allow for the current spike the compressor draws at startup or is this not safe for running other other general stuff in the garage, tools, fans etc?

many thanks
 
Refuse to Bend said:
If ADS is afforded then it is fine. Is the house mcb RCD protected?
Click to expand...
Thanks, yes it is RCD protected... Im assuming i will need to replace both the MCB in the house and the MCB in the garage to C types. i was also wondering why the one in the house trips first?
 
Testing is required with an Earth Loop Impedance tester or Electricans multifunctional installation tester to determine if the requirements are met.
 
  • Like
Reactions: MGW
You can show us all the pictures you like, but the question is what is the loop impedance? A type B MCB will trip on the magnetic part of the trip at 3 to 5 times the thermal trip rating. With a C 5 to 10 times, and D 10 to 20 times.

So a short circuit must draw enough current to ensure the MCB will trip, so a 16 amp type B is 230/80x95% = 2.73Ω and a 16 amp type C is 230/160x95% = 1.37Ω so until the loop impedance is measured, can't answer the question.

Compressors are a problem, as if you reduce the start current too much, they can stall, and often there is a de-loading valve, to allow it to start which is timed, but the use of inverter start is the normal method around the problem of the start current. These use three-phase motors, so not a conversion, need a compressor designed for the supply.
 
You have two 16a B type mcb's and two 30ma RCD's in series on that cct. You would need to resolve this before you replace for a C type as more than likely you will shift the issue from the house CU to the w/s CU. Who ever put this in lacks a basic understanding of how to implement selectivity. You will need to check Zs is within limits.
 
The RCD will provide fault protection regardless of the Zs and assuming the RCD functions and the circuit is sound.
Changing it to a type C may not do the trick as it is only 16A. Without knowing more such as cable size and volt drop it is difficult to advice whether this can be increased in size.
 
Last edited:
Coloumb said:
You have two 16a B type mcb's and two 30ma RCD's in series on that cct. You would need to resolve this before you replace for a C type as more than likely you will shift the issue from the house CU to the w/s CU. Who ever put this in lacks a basic understanding of how to implement selectivity. You will need to check Zs is within limits.
Click to expand...
In fact you don't need the garage CU at all. The house MCB and RCD provides all the protection you need for the garage sockets. For the garage lights replace the switch for a switched FCU with 5A fuse.

I had a switch on surge problem in my garage which was solved by changing the 16A MCB for a 20A one. Whether you can do this though depends on the cable.
 
Mkol said:
below is pics of the MCBs in the main unit in the hosue and the smaller one in the garage.

House:


Garage:
Click to expand...

It sounds easy enough to relace a Type B for a Type C breaker.

However, the earth to live impedance (resistance, if you like) needs to be checked. The maximum allowed on a B16 is 2.15 ohms. On a C16 it goes down to 1.10 ohms. On a D16 (don't do it) it's 0.55 ohms.

It's best to get an electrician to test it. Quite expensive test equipment is needed. Whilst they're at it, they can also sort out some other minor problems you have.
 
malbec4 said:
It sounds easy enough to relace a Type B for a Type C breaker.

However, the earth to live impedance (resistance, if you like) needs to be checked. The maximum allowed on a B16 is 2.15 ohms. On a C16 it goes down to 1.10 ohms. On a D16 (don't do it) it's 0.55 ohms.

It's best to get an electrician to test it. Quite expensive test equipment is needed. Whilst they're at it, they can also sort out some other minor problems you have.
Click to expand...
The RCD can be used for fault protection in that respect exceeding maximum Zs values is in general not important.
 
You must log in or register to reply here.
Back
Top