Circuit protection

[richerdick32]

Hi sounds like a simple question. is ok to use a switch rated at 16a in a circuit protected by a 16 a breaker, or does the switch rating need to be under that of the breaker, the expected load is only 6A

 

[richerdick32]

Sorry i meant to say should the switch rating be over that of the breakers that protects it

 

[jraef]

No, you're fine.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[richerdick32]

Thanks for the reply Jraef. For future reference when i am selecting components like relay/switch contacts etc should I use components that have contact ratings equal to or above the rating of the circuit breaker? or do i only really need to consider the expected load. do you know where this topic is covered in uk wiring regulations?

 

[ScottyUK]

The easy answer, certainly if working under BS 7671, is to ensure the components are equal or greater in capability than the breaker or fuse protectiong them. Obviously there's a sensible limit to that philosophy - there's no good reason to use a 200A switch on a 20A circuit, for example.

I think there's at least one exception in 7671 but unless you're going to take advantage of it dozens of times over I don't think you'd see any saving. Most lighting switches (for example) are designed with a current rating of 10A for use a a circuit with a 10A breaker or fuse because almost every doemstic / commercial / light industrial lighting circuit will use either a 6A or 10A breaker or fuse.

Can you put the question a bit more in context?

 

[richerdick32]

In this instance i want to use a rocker switch rated at 16A to supply a 16A commando socket for an adapter box .The protective device is 16A. I cant find a 20A rocker from our designated suppliers that will cope with the inductive load of the pump i want to control . I have found a 16A one that looks like it will do the trick and want to use that.
Generally i just wanted clarification that i was not making a fuss when i insist that circuit "accessories" such as relays and switches are protected by a device rated either equal to or lower than the current carrying capabilities of the circuit it protects. I know it is true for cables just couldnt find any regulations that backed up my approach when it came to that actual control devices

 

[wroggent]

You may also want to verify that the temperature rating (if known) of your 'accessory' terminals are at least as much as the cable temperature rating for the same ampacity.

 

[ScottyUK]

Bear in mind that BS 7671 allows use of a 20A breaker to protect a 16A BS 4343 socket. Sounds crazy, but true even so. Hope that doesn't mess it up too much.

FWIW, it's usually easier to find rotary switches with decent ratings rather than rockers. You're looking for AC23 rating for a motor switch. Do you have to use a rocker switch?

 

[richerdick32]

Thanks for your help! I have taken your advice and found a suitable rotary switch and was also able to find a small mcb that may or may not be necessary. I had seen that 16A outlets can be protected by a 20A fuse, which seems crazy as it is not unusual to see power tools etc with 0.75 cable on with no fuses in the plugs this seems a bit of a weird approach. But the regs are the regs!

With respect to my wider question, for future reference if I am making alterations in electrical panels replacing components etc it has just highlighted the need for me to clear things up a little.

Do the regulations in BS7671 only apply to supply installations, obviously when designing wiring inside an electrical panel etc you would try to select cables according to their expected load its just that do you need to rate components and cables inside a panel to the size of the breaker that protects them without exception?
To clarify things i will give you a couple of examples these are not specific examples but just highlight my point

Installing a relay in a panel with its coil wired in parallel across an incoming supply cable. the relay being used to drive a light that indicates the presence of supply. The supply cables may be say 6mm so protected b say a 32A breaker, do the coil connections of the relay need to be as big so that they are adequately protected or is this overkill

A phase rotation relay as above connected in line with the main phases, do the sensing cables need to be fused ?

A proximity connected into a control circuit protected by a 4A breaker with a rated switching current of 200mA

I hope from these questions you can see what i am trying to establish, its not something i have thought about much before. I am sure i have come across installations where situations similar to that described above exist.
what is the common practise?

 

[ScottyUK]

7671 is an installation standard, not a panel-building standard, so current ratings and the like in 7671 don't directly apply to the inside of a panel. Generally panel wiring runs at a higher current than stated in BS 7671 for a given conductor size. In answer to your questions, my opinion is -
[ol 1]
[li]The relay should be protected by fuses or a small MCB.[/li]
[li]The phase rotation relay should be protected by fuses or a 3-pole MCB[/li]
[li]Assuming the prox is 24V DC or similar I probably wouldn't sub-fuse it individually unless there was other critical stuff which would be adversely affected if the prox took out the supply bus. The supply should be fused somewhere near the origin, but not necessarily at every load.[/li]
[/ol]