Am I Limited to a 20A Ring In Shed Build?

[JohnW2]

Maybe but the logical outcome of that belief is two (or more) of every such device - clearly not practical.

I'm not suggesting or advocating that, but was really just commented on the irony that some people do sometimes take that approach with RCDs but, seemingly not (or very rarely) with OPDs, despite the latter not being 'testable'.

Mind you, you say that it would be "clearly not practical". In another thread, we are discussing the situation in which every domestic CU in the UK is 'required' to be 'non-combustible' because what I assume to be) a tiny proportion of them (I presume 'tens of millions') are the origins of fires each year (and, even then, probably usually because of poor workmanship, not the fault of the CU). Is such a wide-ranging response to a (I presume) tiny risk any more "practical"?

Perhaps MCBs should contain two OPDs just in case..

Whiklst your tongue was probably in your cheek when you wrote that, it's not an impossible, impractical or even necessarily unreasonable suggestion, particularly in relation to safety devices whose satisfactory functioning cannot be tested.

In some safety-critical situations (e.g. aircraft, nuclear reactors etc.) multiple redundancy is the norm.

Even immersion heaters now include similar 'redundancy'. The thermostat should switch it off if things get too hot, but we now have a ('redundant') secondary thermal cutout, just in case the primary control doesn't do its job. Similar with many other domestic appliances which produce heat.

Kind Regards, John

 

[JohnW2]

I accept that a sudden hard overload from a fault is likely to take out both a 20 Amp MCB at the shed and the 32 Amp MCB in the house but a soft overload ( too many items switched on ) would take out the 20 Amp MCB before the 32 Amp MCB in the house.

Yes, I think that's something which often goes 'overlooked' - that 'discrimination' (even between devices with quite similar Ins) is much greater in relation to what you call "soft overloads" than in relation to (very high current) "faults". Even a 25A MCB should 'discriminate against' a 32A one in relation to a 'slowly increasing load'.

Having said that, you're talking about using OPDs as a means of 'load limitation', rather than cable protection, which is a situation which a designer should attempt to avoid (although his/her abilities to do that are very limited when sockets circuits are involved - unless he/she has a very good crystal ball!).

Whether, in practice, any of this really 'matters' is a different question. Even if the only OPD were a 32A one in the house, then the worst that could happen would be loss of light in the shed, and just a quid or two could provide a battery-powered solution to that (itself very unlikley/rare) 'risk' - and would also address the issue of the shed being plunged into darkness by a power cut or some issue (like an RCD trip) in the house!

Kind Regards, John

 

[AndyPRK]

If it was a loft or garage with no windows then I could understand you wanting to keep the lights on.
But adding in a 20A mcb to trip is a pointless and annoying (when it trips) for a shed with windows when the cables can perfectly cope with more than 20A.


If you were to fit 2 32A MCB's and they both tripped, at least that would help indicate where the fault was.

 

[bernardgreen]

Having said that, you're talking about using OPDs as a means of 'load limitation', rather than cable protection,

No I am not talking about load limitation. I am promoting the idea that the installation should be designed in such a way that overloads should not affect circuits which are not overloaded.

 

[JohnW2]

No I am not talking about load limitation. I am promoting the idea that the installation should be designed in such a way that overloads should not affect circuits which are not overloaded.

I think that's a bit of a semantic issue.

Whether you call it 'load limitation' or anything else, you were talking about a situation in which an OPD in the shed would trip before the one in the house if loads beyond the limit imposed by the shed's OPD were 'plugged in' in the shed. In other words, that shed OPD would 'limit' the amount of 'load current from the shed' that would flow through the house OPD - even if you don't call that 'load limitation'

Kind Regards, John

 

[JohnW2]

If it was a loft or garage with no windows then I could understand you wanting to keep the lights on.

... or, perhaps, it were a shed that one wanted to use during the hours of darkness - but, as I said, for a couple of quid one could buy a battery-powered solution to that potential problem (and it would also still be a solution in the case of power cuts or problems in the house installation).

The same really applies in the case of "a loft or garage with no windows", since the 'battery-powered solution' would still be needed in the case of power cuts or problems with the installation (such as RCD trips)>

Kind Regards, John

 

[bernardgreen]

since the 'battery-powered solution'

Often overlooked is the fact that when a power failure occurs and the normal bright illumination ( mains powered ) is reduced to a lower level ( battery-powered ) of emergency lighting then the human eye takes a few seconds to react ( enlarge the iris ) to provide vision in the reduced lighting.

In most cases the affected person can remain stationary until his or her eyes become accustomed to the reduced lighting and the person can move safely.

If the person is operating machine the sudden loss of vision while the iris reacts could create the risk of injury

 

[JohnW2]

Often overlooked is the fact that when a power failure occurs and the normal bright illumination ( mains powered ) is reduced to a lower level ( battery-powered ) of emergency lighting then the human eye takes a few seconds to react ( enlarge the iris ) to provide vision in the reduced lighting. ... In most cases the affected person can remain stationary until his or her eyes become accustomed to the reduced lighting and the person can move safely. ... If the person is operating machine the sudden loss of vision while the iris reacts could create the risk of injury

You really are scraping the 'ultra-cautious' barrel

However, if you're concerned about that, pay a bit more than £2 and get one of the torches which automatically comes on in the event of a power failure, or pay a bit more than that and get a 'proper emergency light'!

Kind Regards, John

 

[AndyPRK]

You can expect a £2 battery light to have batteries replaced and maintained.

A light hooked up to the mains maybe ok

 

[JohnW2]

You can expect a £2 battery light to have batteries replaced and maintained. A light hooked up to the mains maybe ok

I'm not totally sure what you're saying. I was talking about ones like the below, of which I have several. They obviously have to be 'hooked up to the mains' if they are going to know when there is a power failure, and the (constantly trickle charged) batteries in mine generally last for years.

Kind Regards, John

 

[bernardgreen]

get one of the torches which automatically comes on in the event of a power failure,

and hope it is pointing to space between the circular saw blade and fingers when the lights go out.

 

[davelx]

and hope it is pointing to space between the circular saw blade and fingers when the lights go out.

If you are using the tool correctly there won't be a problem (speaking as someone who has spent many 100's of hours building scenery using such tools)

 

[JohnW2]

and hope it is pointing to space between the circular saw blade and fingers when the lights go out.

Oh my goodness As I said, there are options even for the ultra-risk-averse ...

You really are scraping the 'ultra-cautious' barrel .... However, if you're concerned about that, pay a bit more than £2 and get one of the torches which automatically comes on in the event of a power failure, or pay a bit more than that and get a 'proper emergency light'!

Kind Regards, John