Is a main switch needed on a sub board?

[plugwash]

Suppose one has a 32A circuit but needs two 16A circuits. Perhaps because they want to follow the manufacturers instructions for the cooking equipment. Perhaps they need a couple of 16A commando sockets both capable of running full load.

I see two main options.

1. Install a small CU with two breakers.
2. Install a 2 module enclosure with two MCBs and wire them up. This would effectively form a sub board with no main switch.

Is there any reason not to do 2?

 

[SUNRAY]

Are you saying "Does a FCU have to be a SFCU"?

As long as isolation is possible then No you don't actually need to have a switch.

One only has to read the number of time posters say a cooker/shower/spur doesn't require one. Adding the 16A OCPD is infinitely better that just slapping them on the 32A circuit.

However if this is for 2 ovens etc I'd like to see an accessible isolation device local to the appliances.

 

[RandomGrinch]

Perhaps they need a couple of 16A commando sockets both capable of running full load.

Not that I know anything, but a few of our recently installed machines are plugged into units like this...

 

[AndyPRK]

For a cooker, I would want to be able to isolate the neutral to allow things upstream to still work on an element failure.

 

[EFLImpudence]

Is there any reason not to do 2?

Do you mean as far as the regulations are concerned or do you want everyone's personal preference?

 

[ebee]

My personal preference would be to have an isolator onboard, in other words a 2 way CU.
But if you want to do two MCBs/Fuses and no isolator adjacent then do you think it is somehow compliant/non-compliant with the regs or just not a good idea.
Obviously it should really be properly labelled etc to inform the next person on it to work safely or to remind yourself in a few years time.
Not really that much of a long time ago it used to be very common to see a consumer unit with fuses or breakers, you could see the ratings or colour code and not much else so you had to guess which was the cooker (sometimes it was marked) then the others were power or lighting and you might find 2 or 3 of each but no mention of which lights or which sockets, it was all a guess until you checked.
Happy days.

 

[plugwash]

Are you saying "Does a FCU have to be a SFCU"?

Well that is kind of what drove the question. Noone bats an eyelid at an unswitched FCU or some fuse modules in a grid plate, but anything over 13A nearly always has a main switch. I was wondering if there was something I was missing or if it was fine to just install a MCB or two in a 2-module box to "fuse stuff down".

I posted this in a new thread deliberately to try and decouple this discussion from the discussion as to whether following manufacturers instructions on over-current protection was actually necessary.

Not that I know anything, but a few of our recently installed machines are plugged into units like this...

Yeah, combination units, both single socket variants and multi-socket variants.

At least the single outlet versions seem to ship configured with only RCD protection though, not overcurrent protection.

 

[Murdochcat]

Why would you do option 2?

 

[JohnW2]

My personal preference would be to have an isolator onboard, in other words a 2 way CU.

Would you say something similar about two adjacent (unswitched) FCUs or, indeed, a 'modular' accessory with two fuse modules in it?

If not, what is the difference that you perceive (from two 16A MCBs)?

 

[SUNRAY]

Well that is kind of what drove the question. Noone bats an eyelid at an unswitched FCU or some fuse modules in a grid plate, but anything over 13A nearly always has a main switch. I was wondering if there was something I was missing or if it was fine to just install a MCB or two in a 2-module box to "fuse stuff down".

I suspect there is no specified demarcation. Generally speaking (but not always) tapping from a larger supply such as a ring main or a Henley block would require DP isolation and fusing down and for that reason fuseboxes tend to include a switch. Without knowing full details of your proposition I'd say best practice/commonsense

I posted this in a new thread deliberately to try and decouple this discussion from the discussion as to whether following manufacturers instructions on over-current protection was actually necessary.

I assumed this was the case, If there is an existing circuit including 'cooker switch' feeding multiple cooking appliances in its vicinity and you are looking to add the MI's (or assessed requirement/commonsense) OCP then no additional switching is required. There is of course the option of double pole MCB's if full isolation is considered to be an advantage.

Yeah, combination units, both single socket variants and multi-socket variants.

At least the single outlet versions seem to ship configured with only RCD protection though, not overcurrent protection.

They are available in all sorts of combinations, including empty.

 

[ebee]

Would you say something similar about two adjacent (unswitched) FCUs or, indeed, a 'modular' accessory with two fuse modules in it?

If not, what is the difference that you perceive (from two 16A MCBs)?

Hi John, the difference, to me myself, is that I prefer to have a relatively quick unambiguous way of cutting power whether for emergency, urgency, aid in fault finding or just in normal operation. I almost never use an FCU but rather a SFCU. Exceptions such as a security alarm, fire alarm, stair lifts and a few other things because that might actually invite unintentional consequences in everyday normal use.

Take consumer units as an example, Modern consumer units with MCBs and RCCBs (RCDs) are often used by some as a switch to cut power to particular circuits and are seen as "The Norm".
Back in the days of rewireable fuses being the norm one should never remove or replace without throwing the main Switch, people often did this rather than power down the whole board, remove cover, remove/replace the fuse (or some old fashioned type breaker) then refit the cover then power back up. They would simply pull out and plug in the fuse on a live board and hope it would not bang, flash and splatter molten fusewire all over the place.
Quite often it did and only pure luck would prevent injury. So a switch, if it does not seem likely to cause additional problems in itself, is always my preference.

 

[JohnW2]

Hi John, the difference, to me myself, is that I prefer to have a relatively quick unambiguous way of cutting power whether for emergency, urgency, aid in fault finding or just in normal operation. I almost never use an FCU but rather a SFCU. Exceptions such as a security alarm, fire alarm, stair lifts and a few other things because that might actually invite unintentional consequences in everyday normal use.

Fair enough,and I have to say that,again,n relation to "me myself", I feel and do exactly thee same. It's like when I do plumbing, and people accuse me of having shares in suppliers of isolation valves & 'service valves' etc

However, that's just you and me, and I think it only fair to point out to 'readers' that it's not a matter of regulatory requirements,or necessarily even 'good practice' - but is essentially a 'personal preference'.

As I need not tell you, there are an awful lot of unswitched FCUs ('compliantly') in service. but it's not just FCUs ...

.... whilst many of us choose to have 'isolators'(or SFCUs where appropriate), a good few have things like immersions,showers and kitchen appliances supplied directly from an OPD (including an unswitched FCU) without any means of even SP isolation other than in the CU,with the CU's Main Switch (or RCCB) often being the only means of DP isolation - and, again, the regs seem to have no problem with that.

Kind Regards, John

 

[ebee]

However, that's just you and me, and I think it only fair to point out to 'readers' that it's not a matter of regulatory requirements,or necessarily even 'good practice' - but is essentially a 'personal preference'.

Hi John, yes I pretty much agree with what you have said.
however one light disagreement here though.
My person preferences do often have some bearing on what I believe to be "Good Practice" more often than not.
as I hinted at elsewhere - electrics is simple (well basic electrics can be) just twist two wires together to make something work but doing it "safely" is more complicated.
So we have a set of rules to set some safety standards (The Wiring Regs - started as a set of rules by the society of telegraph and wireless engineers or such, four sheets of A4 written because the started to consider that electric could be dangerous regards fire and electric shock risks).
That evolved to what we have today BS7671.
I believe that, in some respects, that can border on on the bottom end of "good practice" or even perhaps on the top end of not so good practice.
So, some of us have personal preferences that when added give more of a feeling of "good practice".
I do envisage a dividing line where the regs ends and good practice begins, the regs are the minimum we should be working to and could be one step below the start of good practice.
So long as we do not make the mistake of mentally elevating our ideas of lack of "good practice" to be non compliances with what the regs actually require - I have seen this happen a few times over the years and some have become urban myths.

Example - RCDs - back in the mists of time, I was adding RCDs before they had become really popular, I always put them as as an optional extra so as not to skew my estimates as appearing more expensive than others so I did not lose much work. But being an optional extra I would put strong recommendations for customers to have them included.
The regs, at that time, did not require them but I considered it good practice to include them so I usually persuaded them (Not that difficult if you said some like "If you had a pet dog and it saved your dog`s life you would think it worthwhile, well it could save a human life so surely that is worth considering?")

 

[JohnW2]

Hi John, yes I pretty much agree with what you have said. .... however one light disagreement here though. .... My person preferences do often have some bearing on what I believe to be "Good Practice" more often than not.

It may disappoint you to hear that I don't think there is any disagreement there,either "

"Good practice" is vague, and to an appreciable extent subjective, so is essentially "in the eye of the beholder" - and it would be pretty odd if you did not regard your 'personal preference' as being 'good practice'

"Best practice is even worse, and probably rarely exists - since someone will usually come along proposing something 'even better'!

However, if it makes you any more comfortable, there may be some scope for slight disagreement,in some situations The essence which you go on to say uis captured by your ...

....That evolved to what we have today BS7671. .....I believe that, in some respects, that can border on on the bottom end of "good practice" or even perhaps on the top end of not so good practice. .....So, some of us have personal preferences that when added give more of a feeling of "good practice". ....I do envisage a dividing line where the regs ends and good practice begins, the regs are the minimum we should be working to and could be one step below the start of good practice.

I'm not sure whether you intend that as a generalisation but,if you do, I disagree (that it is valid as a generalisation). I would say that in relation to most matters, the 'minimum requirement' of BS7671 seems (to me) to be perfectly 'good practice' and I would not have a 'personal preference'which went beyond that minimum requirement (even when 'going beyond it' were possible!). Only in a very small number of contexts do I feel that 'good practice' (hence my personal preference) requires more than the minimum required by BS7671.

Particularly if one regards "not-so-good" as essentially meaning "somewhat bad", I can think of very few things in BS7671 which I would regards as being "... perhaps at the top end of not-so-good practice". In general, I would say that the minimum requirements of BS7671 do represent 'good practice',and that the 'good practice' you talk about (and which drives your personal preferences) would probably better be described as 'better practice'.

Although we can't do anything about it, I personally think that there are some of gthe requirements of BS7671 which go beyond what I would personally be happy to regard as 'good practice'.

Kind Regards, John

 

[ebee]

John, could we come nearer to aligning of agreements is we rethought "good practice" as "fairly good practice (not bad practice) " and best practice as a better practice ?