Two circuits off cooker switch?

[JohnW2]

If say the cable is rated at over 50 amps, and the fuse is 50 amps, and the appliance is rated at say 15 amps, should we be concerned how small the appliance internal wiring is?

We should, given that manufacturers often (usually?) fail to include internal protection for the innards of their products. This is presumably why they often specify a seemingly irrationally low rating of OPD (like fans on 5A/6A circuits and wires with 1mm² cable requiring 3A FCUs). However, if they don't specify such a requirement, how can we know what is inside their machines, or what sort of protection may be required?

Kind Regards, John

 

[sparkwright]

If say the cable is rated at over 50 amps, and the fuse is 50 amps, and the appliance is rated at say 15 amps, should we be concerned how small the appliance internal wiring is?

We should, given that manufacturers often (usually?) fail to include internal protection for the innards of their products. This is presumably why they often specify a seemingly irrationally low rating of OPD (like fans on 5A/6A circuits and wires with 1mm² cable requiring 3A FCUs). However, if they don't specify such a requirement, how can we know what is inside their machines, or what sort of protection may be required?

Kind Regards, John

Would it be fair to say then that for every load we should always fit an overcurrent device equal or slightly above the total current demand of the load?

And is it fair to say relying on the just the supply cable being big enough is not all we should consider?

 

[JohnW2]

Would it be fair to say then that for every load we should always fit an overcurrent device equal or slightly above the total current demand of the load?

Ooooh! Doesn't sound 'unfair' to me - but last time I said something like that here (I have a good few FCUs with 1A fuses in them, despite 1mm² cable), the flak didn't die down for days The 'official line' is that OPDs are there to protect fixed cables of the installation (and maybe the flex of hard-wired loads) and nothing else - and that if the innards of the load requires protection, that protection should be provided within the load.

.. And is it fair to say relying on the just the supply cable being big enough is not all we should consider?

Again that doesn't sound very 'unfair' - but, as I said before, one can't really 'consider' things inside the load/appliance that we don't know about - we have to rely on the manufacturer specifying what OPD is need to protect the load in such situations.

Kind Regards, John

 

[sparkwright]

To be in line with the manufacturer's instructions, you could

Run one 6 mm2 cable from the wall switch to a new 2 way consumer unit fitted in a nearby kitchen cupboard.

At the 2 way board, fit one 20 amp MCB and one 16 amp MCB (as desccribed in the MI, right?).

Then from the new board run one 2.5 mm2 to one appliance,

and from the new board run another 2.5 mm2 to the other appliance.

Would it be fair to say this kind of set up would satisfy every problem?

 

[JohnW2]

To be in line with the manufacturer's instructions, you could
Run one 6 mm2 cable from the wall switch to a new 2 way consumer unit fitted in a nearby kitchen cupboard.
At the 2 way board, fit one 20 amp MCB and one 16 amp MCB (as desccribed in the MI, right?).
Then from the new board run one 2.5 mm2 to one appliance,
and from the new board run another 2.5 mm2 to the other appliance.

Would it be fair to say this kind of set up would satisfy every problem?

Probably/possibly - although, if one wants to pay it totally by the book (strictly complying with MIs), I've been getting a bit confused by varying statements about what the MIs do and do not exactly say as regards minimum and/or maximum OPD ratings.

Kind Regards, John

 

[sparkwright]

I think some MI can be inaccurate or misleading, particularly if written by a foreign company. I think the bit about minimum fuse size could be a mistake. That said, even a maximum fuse size wouldn't be the correct thing to put either.

Depending on what country, the actual fuse size should be stated. Thankfully in Europe at least, it seems most circuit breakers are similar to ours.

 

[JohnW2]

I think some MI can be inaccurate or misleading, particularly if written by a foreign company. I think the bit about minimum fuse size could be a mistake. That said, even a maximum fuse size wouldn't be the correct thing to put either.

Agreed. The problem in some people's eyes is that strict compliance with the current version of BS7671 seems to require total compliance with MIs, whatever they may say! Hopefully, the proposed changes for next year will allow a lot more discretion and common sense to be exercised.

Depending on what country, the actual fuse size should be stated.

That's fair enough so long as one is going to have a separate OPD for each load - but, of course, that's often not how we are used to doing things in the UK.

Kind Regards, John.

 

[ban-all-sheds]

If say the cable is rated at over 50 amps, and the fuse is 50 amps, and the appliance is rated at say 15 amps, should we be concerned how small the appliance internal wiring is?

Not unless the manufacturer gives us cause for concern.

 

[ban-all-sheds]

Which cable - trhat in the fixed wiring of the circuit or the flex of the appliance (if it has one)?

If it has one then the person who installs the appliance must consider the size and type of it when deciding what type of protective device is suitable.

Simples.

 

[JohnW2]

Which cable - trhat in the fixed wiring of the circuit or the flex of the appliance (if it has one)?

If it has one then the person who installs the appliance must consider the size and type of it when deciding what type of protective device is suitable. ... Simples.

"Simples" in common sense terms, yes - but, per current regs, "Simples" only if the MIs do not make statements which, strictly speaking, prevent the designer/installer from deciding what device is suitable.

Kind Regards, John

 

[ban-all-sheds]

Anybody who thinks that a manufacturer instruction which contravenes other regulations has to be followed should not be doing any design or installation work.

 

[JohnW2]

Anybody who thinks that a manufacturer instruction which contravenes other regulations has to be followed should not be doing any design or installation work.

I'm inclined to agree, but it's clearly wrong that the regs, as currently written, can sometimes require people to contravene one regulation in order to comply with other(s). The (hopefully) upcoming change should make life easier for those who are worried about (or 'forbidden' from - by employers, insurers, scheme operators etc.) contravening any regulation.

Kind Regards, John