Fusing down

Joined
20 Aug 2009
Messages
10,176
Reaction score
1,435
Location
Dorset
Country
United Kingdom
If for example it's acceptable to wire in a 2.5 kW oven (with 1.5 mm ² flex already on it) into a cooker outlet on a 32 amp 6 mm ² circuit without using a fused spur with a 10 or 13 amp fuse in it...

...is it therefore acceptable to connect a 120 watt towel rail (with 0.75 mm ² flex already on it) into a 32 amp 2.5 mm ² ring circuit without using a fused spur, just a double pole switch?

My question is purely hypothetical. I would always use a fused spur unit.

But I understand both are fixed loads, and cannot be overloaded.

What, if any, are the differences?
 
Sponsored Links
So everytime we fit a fused spur off a socket circuit to supply something like a ceiling rose we don't necessarily have to?

Granted most things do come with instructions. For example a wall light will often stipulate a fuse size.

What kind of things could be wired into a socket circuit without a fused spur then?
 
So everytime we fit a fused spur off a socket circuit to supply something like a ceiling rose we don't necessarily have to?
It saves you having to work out if the cable can handle any fault current or if the load may cause an overload.

Granted most things do come with instructions. For example a wall light will often stipulate a fuse size.
You will soon perhaps have or be able to decide if they are valid.

What kind of things could be wired into a socket circuit without a fused spur then?
As above.
What did you have in mind?
 
Sponsored Links
Granted most things do come with instructions. For example a wall light will often stipulate a fuse size.
You will soon perhaps have or be able to decide if they are valid.
Sort-of true, but if remains true to the draft, next year's amendment to the regs will still say that the manufacturer's instructions "should be taken into account". Those who do not wish to, or cannot, think or make decisions/judgements may continue to interpret that as an invitation to 'blindly follow' the MIs.
What kind of things could be wired into a socket circuit without a fused spur then?
As above.What did you have in mind?
It's probably easier to provide answers to the opposite question. For example, I would not be happy saying that an appliance/ equipment 'could not create an overload' if it contained any motors.

Kind Regards, John
 
What did you have in mind?

Well, as I said, anything that doesn't come with MI stipulating fuse sizes. A lot of things to have fuse sizes given now.

An example could be an old house that some one has added lighting for a single storey extension just wired into the back of a socket. Three pendants. Lighting in 1.0 mm ² cable.

When I across this I fitted an unswitched fused spur with a 5 amp fuse as it felt the correct thing to do.

I suppose a pendant is assumed to be 100 W, though 150 W or so lamps are (were) available. I suppose you can only overload a pendant depending on what lamps will physically fit into it.

Does this scenario need a fused spur? By instinct I would fit one.
 
When I across this I fitted an unswitched fused spur with a 5 amp fuse as it felt the correct thing to do.
So would I - saves having to work out anything.

I suppose a pendant is assumed to be 100 W, though 150 W or so lamps are (were) available. I suppose you can only overload a pendant depending on what lamps will physically fit into it.
Does this scenario need a fused spur? By instinct I would fit one.
I would too.
Isn't there a 16A limit for lighting circuits. What is a lighting circuit?

Pendants, switches etc. may only be rated at 6A but may normally only have to handle half an amp but a lot more when the lamp blows.

Perhaps the omission of overload protection should only be used when actually advantageous for other reasons.
 
Can you think of an example of supplying something other than a socket from a socket circuit without using a fused spur?
 
I would not be happy saying that an appliance/ equipment 'could not create an overload' if it contained any motors.
I agree, but that is not contrary to what I wrote, is it?
Not at all. However, you missed out the 'for example' and previous sentence which preceded my words which you quoted - thereby somewhat losing the sense of the simple point I was trying to make - namely that's it's probably easier to come up with things which we feel might be able to produce an overload than ones which we are 'certain' could not.

Kind Regards, John
 
2.5kW oven :)

Electric fire ?

In customers' houses it is probably better to always fuse down on things they could alter.
 
I would too. Isn't there a 16A limit for lighting circuits.
There is.
What is a lighting circuit?
Good question. However, if, as it appears, you're trying to suggest that, by inserting an FCU between a 32A sockets circuit and 'lighting wiring', you would be creaating a 'new' (13A) 'lighting circuit', then that would open up a whole can of worms - since, as we've discussed before, it would be at risk of making any addition to an installation which involved an FCU into notifiable work!
Pendants, switches etc. may only be rated at 6A but may normally only have to handle half an amp but a lot more when the lamp blows.
True, but that's more akin to a fault than an overload (and is certainly very short-lived). If we had to use accessories 'rated' to carry PFC, things could get pretty interesting :)
Perhaps the omission of overload protection should only be used when actually advantageous for other reasons.
I think one might struggle to come with reasons which sounded particularly 'advantageous', since it's generally a matter of convenience/laziness/cost (i.e. avoiding installing overload protection or fatter cable), mightn't one?

Kind Regards, John
 
There is a 3 meter rule between point where you tap into a supply and where you put a fuse.

To me in a switch room with buzz bars you could hardly wire a new fused isolator in 300mm cable when it is going to have a 32A fuse 4mm would be ample as there is nothing in that room likely to damage the short length of cable between the buzz bar and the fuse.

There is debate as to if that same rule applies to an unfused spur where the plug is the point where it's fused down. It could be argued a unfused spur should not exceed 3 meters.

Lighting circuits are another odd one. In the main a BA22d lamp holder is rated at 2A yet we feed from a 6A MCB. Theory is every bulb should have it's own built in fuse. The ceiling rose however could feed many more lamps so since the humble ceiling rose is rated 6A then that limits the MCB size for lights to 6A.

A cooker is another odd one. We have traditionally feed cookers with a 32A feed so using a double cooker connection unit and feeding hob and oven from the 32A supply is normally considered OK but clearly the supply cable does need to be able to carry the full 32A so it will auto disconnect under fault conditions. However when using a 45A supply then one does need to think a bit harder. There is no fuse in the oven and even if the cable will take 45A unless the manufacturer says it's OK then that is stretching it a little too far.

I have felt for some time there is a need for a 16A fuse connection unit to allow us to use equipment designed for Europe but if there is such a beast I have not seen one.

Is it acceptable to wire a table lamp with 0.75mm flex even with a 13A fuse well likely the answer is yes as less than 3 meter unlikely to get the cable damaged and the bulb has built in fuse. But even if towel rail same wattage as bulb to wire that in 0.75 flex would not be acceptable as no fuse in the towel rail.

There is no reason why an oven manufacturer should not include a blue 16A cartridge fuse in the oven in which case wiring in 1.5mm would be OK. But in the main they don't include a fuse so from 32A supply at least 4mm required.

Fixed loads can have an over load situation under fault conditions and with an oven when the element fails it clearly can produce an earth fault so all cable to the element must be able to carry that fault current long enough for the supply to auto disconnect.

Where we hit a grey area is with RCD protection. Although I can see where an oven under fault conditions may cause current to the protected limit to flow to earth I can't see how over design current could flow line to neutral except with mechanical damage. So with RCD protection it could be argued even a 45A supply would be OK.

It really comes down to a risk assessment and if we follow convention then in the main we have no need to show we has done a full risk assessment however once we deviate from convention then it's clearly up to us to do the risk assessment as for example with a 45A rather than 32A supply to cooker.

There are so many things we do which don't really follow the rules but we have done them for years and no one queries if they comply.

Everyone uses a dedicated supply to the immersion heater and in the main also to built in oven but the load to a tumble drier is likely higher than that of an oven and clearly over the weight to be called portable so should have dedicated supply same with washing machine and dishwasher but convention is we just plug into the kitchen ring. I suppose it could be argued the kitchen ring is a dedicated supply.

For years DIVISION OF INSTALLATION was not considered to cover the RCD then in 2008 it was made plain it did include the RCD and when the regulations clarify rather than introducing something new then the question is do we need to fail or report where where it does not comply?

The English used in the regulations is clearly carefully selected but when we interpret the regulations we tend to bend the rules. One was "Complies with previous edition" before 1992 there was no BS7671 so lights without earth do not comply with a previous edition of BS7671 and `Regulations for Electrical Installations' came out in 1981 before that it was called `Regulations for the Electrical Equipment of Buildings' so lights without earths do not comply with a previous edition!

Personally I think we should not be trying to bend the regulations to suit what we want we should be instead looking at the intent, and what makes common sense.

The fact that the guide and the regulation are now separate documents does not mean we should ignore what's written in the guide or follow what has been convention for years.

I know gas cookers have no isolators within sight of the operator, and in the case of emergency you have to turn off the whole gas supply. But we are tradesmen, and as such putting an isolator in view of the operator, where it will be assessable in the case of an emergency (i.e. not in a cupboard or where a burning chip pan would stop you using it.) makes sense, and should be provided even if it's no longer required by regulations.

We should not need the regulations we should be doing all what the regulations ask for and more without being cajoled into doing it. We are tradesmen and should behave as such.
 
Pendants, switches etc. may only be rated at 6A but may normally only have to handle half an amp but a lot more when the lamp blows.
True, but that's more akin to a fault than an overload (and is certainly very short-lived). If we had to use accessories 'rated' to carry PFC, things could get pretty interesting :)
I suppose it is.
So is it the switches and lampholders which may be damaged by this current even if the cable would not?

Perhaps the omission of overload protection should only be used when actually advantageous for other reasons.
I think one might struggle to come with reasons which sounded particularly 'advantageous', since it's generally a matter of convenience/laziness/cost (i.e. avoiding installing overload protection or fatter cable), mightn't one?
Well - if you like but if it complies with the omission of overload regulations it could also be considered advantageous.
I am primarily thinking of the oven installations.

If not, what is the reason for the regulation other than it is safe?
 
There is a 3 meter rule between point where you tap into a supply and where you put a fuse.
That's often said but, in practice, AFAICS it is not often (probably hardly ever) going to be true.

433.2.2 allows one to have a protective device (fuse or whatever) downstream of a reduction in CSA if at least one of two conditions is satisfied. The second of those is, indeed, that the distance (from CSA reduction to OPD is less than 3m ('and is installed in a manner such as to minimise risk' etc.). However, the other (first) of the two conditions is that there should be adequate fault protection for the cable in question.

As we recently discussed at length, surprisingly small conductors get adequate fault protection from quite large MCBs. In practice, therefore, it will nearly always be the case that the cable between CSA reduction and downstream OPD is provided with adequate fault protection, in which case it is seemingly not necessary to also satisfy the 'not more than 3m' condition.

At least, that's how I read the regs - do you disagree?

Kind Regards,
John
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top