Heating an airing cupboard

[PBC_1966]

Do you mean the one I answered here?

{....}

BS 7671 does not say that attempts to subvert the design of the installation because the manufacturer has been unable to make a product which is intrinsically safe should be allowed.

How is it "subverting" the design of the installation? The equipment in question is a part of the installation.

What are you going to try and claim next? That a manufacturer shouldn't design a piece of equipment which draws 40A because it "subverts" your design of the rest of the installation by requiring something more than the 1 sq. mm cable you'd like to use throughout?

The wiring and associated switchgear are there to serve the appliances, and thus it's not possible that selection of those appliances can do anything but have some effect on the requirements of the rest of the installation. That's not subverting the design of the wiring, it's a natural consequence of the wiring being there to serve the appliances.

I have a gas water heater in the garage, for example, the manufacturer's instructions for which state that a temperature & pressure relief valve of whatever rating maximum must be fitted to protect against the possibility of the thermostat becoming faulty or some other problem resulting in an excessive build up of pressure inside.

Fitted where, to what?

Fitted to the appropriate boss provided for it on the water heater.

Is that wrong? Has the manufacturer built something which is "so intrinsically unsafe" that it has to rely on that external device for protection?

Is it relying on that device for protection of itself against its own faults?

Of course it's for protection against certain faults within the water heater - What do you think a T&P relief valve is there for?

If I were to remove the T&P relief valve and blank it off, "because the manufacturer shouldn't rely on it" and then a faulty thermostat resulted in the water heater exploding, do you think it would it be the manufacturer's fault for relying on that external device, or my fault for ignoring his safety instructions?

Why do you think that the maker should not integrate the necessary temperature & pressure relief valve into his equipment?

Why do you think he shouldn't specify that it be fitted by the purchaser during installation?

Why do you think that gas water heaters and pressure vessel protection are analogous to MCBs?

Why do you think they're not? (At least that's what you seem to be implying.)

 

[Risteard]

May I wire a 40A 'bulb' with 10mm² cable and 63A opd?

Depends what you are connecting to it. BS 7671 requires that certain luminaires be connected to a circuit with a rating not exceeding 16A.

 

[ban-all-sheds]

How is it "subverting" the design of the installation? The equipment in question is a part of the installation.

The other parts of the installation supply it with power. They have to be able to supply it with power at the appropriate voltage and frequency, and they have to be capable of delivering sufficient current.

Supplying the wrong voltage or frequency might damage the equipment, but having the capability to supply more current than the equipment needs will not, any more than a road capable of sustaining a car travelling at 150mph will damage a car which is only designed to do 100.

What are you going to try and claim next? That a manufacturer shouldn't design a piece of equipment which draws 40A because it "subverts" your design of the rest of the installation by requiring something more than the 1 sq. mm cable you'd like to use throughout?

I think I'll claim that I will leave it to others to say if they find that analogy useful and accurate.

The wiring and associated switchgear are there to serve the appliances, and thus it's not possible that selection of those appliances can do anything but have some effect on the requirements of the rest of the installation.

That is true. If an appliance draws 10A then the rating of the OPD must be at least 10A, and the CCC of the circuit cable must be at least 10A.

That's not subverting the design of the wiring, it's a natural consequence of the wiring being there to serve the appliances.

It is subverting it as soon as the appliance maker says that the rating of the OPD must be no more than xA.

10 ≤ 32 ≤ 40. There is nothing wrong with that and the maker has no right to enforce 10 ≤ 3213 ≤ 40, for example.

Fitted to the appropriate boss provided for it on the water heater.

So that would be analogous to providing a fuseholder built into the equipment and specifying the rating of the fuse to go in that. No problem there.

Of course it's for protection against certain faults within the water heater - What do you think a T&P relief valve is there for?

If it's for protection against certain faults within the water heater then it should be integrated with the water heater.

Why do you think he shouldn't specify that it be fitted by the purchaser during installation?

An electric shower has an over-temperature cutout integrated into it. They don't ask the installer to provide that safety device himself. It also has a pressure relief device, they don't ask the installer to provide that either.

As supplied, the shower contains safety devices designed to prevent danger arising from internal faults. So it should be with fuses.

Why do you think that gas water heaters and pressure vessel protection are analogous to MCBs?

Why do you think they're not? (At least that's what you seem to be implying.)

Where are they located? At the source of the water and gas supplies which are connected to the appliance, like an MCB, or closely coupled to the appliance itself?

 

[JohnW2]

Supplying the wrong voltage or frequency might damage the equipment, but having the capability to supply more current than the equipment needs will not...

Not during normal functioning. However, should a fault in the equipment result in the flow of an excessive current, the greater the capability of the circuit to provide current (limited by an OPD), the greater may be the damage sustained by the equipment as a result of the fault.

Kind Regards, John

 

[ban-all-sheds]

And in that case the root of the problem is not that the circuit is suddenly forcing more current through the appliance, the root cause is the fault in the equipment, so if that is something which needs to be guarded against then the provision of something to limit the current which the faulty equipment can draw should be made in the equipment.

 

[PBC_1966]

And in that case the root of the problem is not that the circuit is suddenly forcing more current through the appliance, the root cause is the fault in the equipment, so if that is something which needs to be guarded against then the provision of something to limit the current which the faulty equipment can draw should be made in the equipment.

And what if the cause of the appliance suddenly drawing more current is due to a fault in the supplying installation which causes the applied voltage to the appliance to increase?

 

[JohnW2]

And in that case the root of the problem is not that the circuit is suddenly forcing more current through the appliance, the root cause is the fault in the equipment, so if that is something which needs to be guarded against then the provision of something to limit the current which the faulty equipment can draw should be made in the equipment.

Ideally, probably yes, but an alternative strategy is to require users to provide that protection externally.

Why is it that it is almost universal for domestic (and many other) lighting circuits to be protected by 5/6A OPDs, when the cables could often be adequately protected by a 16A or 20A OPD? If, as is commonly suggested, it is because of the 'ratings' of lighting accessories and fittings, do you feel that they (e.g. ceiling roses, switches and light fittings) should all come with internal fusing appropriate to their 'ratings'?

Kind Regards, John

 

[stillp]

Consumer units, and their main switches, are intended to be protected by a cutout fuse of not greater than 100A.

 

[JohnW2]

Consumer units, and their main switches, are intended to be protected by a cutout fuse of not greater than 100A.

Indeed. Would it be fair to say that they are "required" to be so protected (externally)?

Kind Regards, John

 

[stillp]

If they claim to meet BS EN 61439-3 then they are.

 

[ban-all-sheds]

And what if the cause of the appliance suddenly drawing more current is due to a fault in the supplying installation which causes the applied voltage to the appliance to increase?

Were that to happen then everything supplied by that installation is at risk.

I hear the distinct sound of straws being clutched.

 

[ban-all-sheds]

Ideally, probably yes,

No, not "ideally" or "probably". That is the way, and that is the only acceptable way.

but an alternative strategy is to require users to provide that protection externally.

Another alternative strategy is to not use electricity.

Why is it that it is almost universal for domestic (and many other) lighting circuits to be protected by 5/6A OPDs, when the cables could often be adequately protected by a 16A or 20A OPD? If, as is commonly suggested, it is because of the 'ratings' of lighting accessories and fittings, do you feel that they (e.g. ceiling roses, switches and light fittings) should all come with internal fusing appropriate to their 'ratings'?

No.

Either you are unable to see the similarities between the copper inside accessories and the copper inside cables, and the difference between accessories, cables, and current using equipment, or you are about to embark on one of your tedious arguing-for-the-sake-of-it campaigns.

Time will tell which.

 

[ban-all-sheds]

If they claim to meet BS EN 61439-3 then they are.

And so are the cables connected to them.

Cables and accessories may well have to carry the current for a varied and indeterminate collection of loads. An individual load does neither, and so what is done for the first class does not have to be done for the second.

 

[PBC_1966]

One part of an installation often depends on another part of the installation for its safety, due either to a fault within that part or a fault elsewhere. That's how it works in the real world.

 

[ban-all-sheds]

How it should work in the real world is that people should make things which contain all the features necessary to mitigate faults which are confined to those things.