Basic Circuit-Breaker Questions

[Nigel1951]

Firstly don't worry - I'm not doing DIY consumer unit installation! This is just for my education.

I want to make sure I fully understand how circuit breakers are wired in a standard single-phase UK 240v installation.

Am I correct that single-pole breakers are used for circuit protection whereas 2-pole breakers are used where, additionally, isolation is required?

For single-pole breakers am I correct that they are always in-line with the live ("brown") wire, between the supply and the appliance(s) being protected?

For 2-pole breakers am I correct that they are always in-line with both the live ("brown") and neutral ("blue") wires, again between the supply and the appliance(s) being protected?

Is it normal for all circuits to use single-pole breakers unless there is a reason to want to be able to isolate the circuit, or are there some types of appliance that must always have 2-pole breakers?

I have heard of 3 and 4 pole breakers.  Am I right in assuming these are only for multi-phase circuits?

Many thanks!

Nigel

 

[steptoe]

oh,

and then the argument started,,,,,,,,,,,,,,

there is a lot of disagreement as to whether SP devices are adequate or not, and depends on the earthing system also as to what type are acceptable,

but,

we all like a good discussion,.

:slap

 

[Joseph Howarth]

3 pole's are used for 3-phase isolation and certain bathroom fans that have a constant live, trigger live and neutral, 4 pole devices are used for TP-N (L1+L2+L3+N) isolation

 

[Evans Electric]

Just trying to think about DP  MCBs    ...electrode water heaters is one use for them , where all poles should be opened.

Single pole always "in line" as you say, always in the live ...nothing else.

The 3 pole ones are for as you say , for 3 phase circuits .

3 pole's are used for 3-phase isolation and certain bathroom fans that have a constant live, trigger live and neutral, 4 pole devices are used for TP-N (L1+L2+L3+N) isolation

Joseph,  the fan one is just an isolator switch , not an MCB . 

 

[Joseph Howarth]

Joseph,  the fan one is just an isolator switch , not an MCB . 

Oh yeah just realised that now you've said it :facepalm:   ha, forgot it said MCB!

 

[Evans Electric]

No probs.

 

[Nigel1951]

Thanks folks.  The background to this is I'm having a small industrial cabinet built to house a number of PSUs (all 240v in and 24v or 12v out, all fairly low power (one is 24v/20a, the rest are 12v/1.3a).

I'm gett9ing conflicting advice from my normal domestic electrician and an industrial electrician.  One view is that I need separate circuit breakers for each PSU but they can be single pole provided the supply to the cabinet has an isolator; another is the same but the PSU breakers should be 2-pole and a third view is that I only really need a single 2-pole breaker on the main supply feed to the cabinet, rather than on the supplies to each PSU.

I gather any of these are permissible under the regs, just wanted views on which is best practice.

The cabinet is for personal hobby use (it will go in a small observatory and the PSUs are to power the telescope mount (the 24v/20a one) and small items (the 12v/1.3a ones).  The cabinet I'm using will be locked with the breakers (if I have any) on the front door, under a transparent lid.

Nigel

 

[steptoe]

ok, my thoughts on this are,

how much earth leakage are on the PSU's ?

I'd be wary of having too many on one MCB,

personally,

I'd like to see radials on DP RCBOs as the best option.

 

[Nigel1951]

Sorry Steptoe you'll have to translate that a bit for me! How is earth leakage determined and when you say radials with DP RCBOs are you saying I should insist on separate circuits for each PSU with DP breakers on each?

 

[Sidewinder]

It is doubtful that a control cabinet should be built to BS7671 anyway, as it does not cover control cabinets only building wiring.

If you are to build the cabinet to relevant standards for correct compliance etc. then each power supply will need DP protection on both sides within the panel.

The supply to the panel will need to comply with 7671.

The panel will need suitable overall isolation.

This is not an area covered in 7671 as it is not relevant to 7671.

There could be other issues that you have not mentioned, such as its intended use, what the power supplies are powering etc.

 

[Nigel1951]

Sounds a bit like overkill for my personal use. In theory I could use standard consumer-use PSUs and just plug them into 13a sockets on the domestic ring, but because the kit will be used in an unheated environment and is expensive I'm wanting to use industrial PSUs because of the wider operating ranges and better voltage regulation. That means I have to stick them in a lockable cabinet (because they have exposed terminals). However if the fact I'm using a cabinet takes me into the realm of industrial standards and testing regimes then it is way over the top for my little application.

I'll need to clarify this a bit more before proceeding.

 

[Sidewinder]

If it is for your own personal use, not in conjunction with any kind of business undertaking of any description, including clubs and charities, and for use in your own home, then do as you like, just ensure that you comply with the requirements of your house buildings & contents insurance and you are fine.

In the event of an issue, you would need to be able to justify the design & manufacture of the bespoke unit was "suitable & sufficient" to your insurer that is all.

That is if the thing went wrong & burned the house down!  :innocent

Plus you would need to comply with the equipment manufacturers instructions for warranty, and if industrial type equipment, then I doubt they are cheap, and the makers would expect them to be installed in line with industrial standards etc. :coat

HTH,

 

[Nigel1951]

Thanks, sounds like the way to go. I'll build it with informal support and checking from my electrician but without going through the formalities as it will be installed in an outbuilding (with a dedicated supply from the main consumer unit which my electrician will install) and is for my personal use.

Nigel