Garage supply

[trojanhawrs]

Sure - but, as you imply, it's the Ze that is the saving grace.

Indeed, we might have derailed a bit but hopefully it'll help to convince the OP that this is a job for an electrician.

 

[JohnW2]

Indeed, we might have derailed a bit but hopefully it'll help to convince the OP that this is a job for an electrician.

Maybe, but he might want to take note of the fact that I am not, never have been and never will be, an electrician!

It's what one knows that counts/matters, not whatever label one may (or may not) have attached to oneself

 

[trojanhawrs]

Maybe, but he might want to take note of the fact that I am not, never have been and never will be, an electrician!

It's what one knows that counts/matters, not whatever label one may (or may not) have attached to oneself

You need to know how to apply that knowledge as well though, and I guarantee if you've not run in and made off a 16mm 3 core SWA before you will make a **** of it. Circuit design is important but that includes physically fitting it as well.

 

[JohnW2]

You need to know how to apply that knowledge as well though, and I guarantee if you've not run in and made off a 16mm 3 core SWA before you will make a **** of it. Circuit design is important but that includes physically fitting it as well.

All very true. However, although you suggested that our rather tangential discussion may have "helped to convince the OP that this is a job for an electrician", the discussion had actually been entirely about 'design', rather than the practical skills needed to actually execute the work!

 

[ebee]

Indeed - provided, of course, the OPD has an adequate rating for the anticipated loads.

I think your (2) is probably redundant. Since no cable has a neutral with a CSA less than that of the CPC, if the circuit's 'maximum Zs' is satisfied [i.e. your (3) ], then (2) would also inevitably be satisfied.

Kind Regards, John

I dont disagree with you John, i am just quoting the three basic considerations we must cover by one means or another and these are the initial considerations we are taught 1 or 2 or all 3 of them might be covered by the same OPD or indeed by seperate devices or also by the nature of the load. Then we go on to other considerations like volt drop etc.

Indeed we might need an RCD to cover Earth fault anyway or in a TT system or for additional (supplementary) protection anyway.
My own preference is to have a TN system if possibe and the OPD taking care of earth faults with supplementary protection via RCD usually but sometimes you might have to have an alternative method.
If it needs sole reliance on an RCD working then I prefer two in series and preferably in different environments and preferably of different makes in order to reduce chances of the ill effects of "stiction" although that might mean cascading with a RCD that cant be relied upon for personal protection (certainly not if the fault occours whilst touching, although if tge fault occurs prior to touching then, depending upon the nature of said fault, it might in some cases, disconnect prior touching it)

 

[JohnW2]

I dont disagree with you John, i am just quoting the three basic considerations we must cover by one means or another and these are the initial considerations we are taught 1 or 2 or all 3 of them might be covered by the same OPD or indeed by seperate devices or also by the nature of the load. Then we go on to other considerations like volt drop etc.

Fair enough. The one thing I forgot to say was that my comment (obviously) related to TN installations - i.e. when primary fault protection was provided by an OPD.

However, whilst I agree that, conceptually, one has to 'consider' the three 'protections' you mentioned (overload, short-circuit and fault), I don't really understand what 'considering' one can do in relation to short-circuit protection. In fact, BS7671 does not appear to specify any specific 'requirements' in relation to overload protection, other than requiring that the In of the OPD is low enough, otherwise relying on the device complying with ('performance-wise') the relevant product Standard. A 'short-circuit' is simply an extreme case of overload, so the same applies - so, in addition to my previous comment, if 'requirements' for overload protection are satisfied (i.e. In is low enough), then presumably the requirement for short-circuit protection must surely be similarly satisfied?

In fact, the more I think about it, the more I wonder why we talk separately about short-circuit protection, given that, as above, a short-circuit is simply the most extreme possible case of an 'overload'

My own preference is to have a TN system if possibe and the OPD taking care of earth faults with supplementary protection via RCD usually but sometimes you might have to have an alternative method.

That's obviously the (historical and current) conventional approach (in TN installations) but I do wonder whether it remains a sensible way of thinking today,given that RCDs are dramatically 'better' at detecting (and clearing) L-E faults than are OPDs - and LE fault resulting in a fault current of, say, 100 mA might be enough to kill, yet if ADS was provided by, say, a B32 MCB, the fault current might have to be as high as 160,000 mA before the fault was cleared.

In fact, given that the Zs of real-world circuits is often close to the 'maximum for ADS (with required disconnection time)', I have to wonder how often OPD-mediated ADS actually 'works' in the face of real-world L-E faults -since those faults have to be very close to 'negligible' (i.e. nearly zero) impedance for the ADS to work with the required disconnection time.

Since RCD protection is now required for almost all circuits, I suppose it doesn't make much difference, but I do wonder whether our thinking should now be that fault protection is provided primarily by RCDs? One might extend that thinking to say that the OPD provided 'additional protection' (i.e. turning current thinking on its head), but I'm not sure that would be very useful - and if we came to think that fault prottection was only provided by RCDs (as below, maybe with RCD redundancy), then we would lose any requirements regarding Zs of circuits, which could be beneficial!

If it needs sole reliance on an RCD working then I prefer two in series and preferably in different environments and preferably of different makes in order to reduce chances of the ill effects of "stiction" although that might mean cascading with a RCD that cant be relied upon for personal protection (certainly not if the fault occours whilst touching, although if tge fault occurs prior to touching then, depending upon the nature of said fault, it might in some cases, disconnect prior touching it)

I certainly never knock the concept of redundancy of protective devices/measures (of any sort) but it's interesting to note that we often see people being 'criticised' for putting RCDs in series, but virtually never see people suggesting that one should put them in series! What seems (to me) to be rather illogical is that it is nevertheless far from unknown to see people (like you above, and sometimes me!) advocating redundancy of RCDs, but never advocating redundancy of MCBs - despite the fact that RCDs are easily testable whereas MCBs are (in practice) essentially untestable (and quite possibly as prone to 'stiction' as are RCDs)!!

Kind Regards, John

 

[trojanhawrs]

Overload and fault protection is primarily there to protect against fire. Same goes for 100mA RCDs on TT systems, it's not there primarily to protect against electric shock and is not there for redundancy, it's ensuring ADS in a L-E fault on the supply cable/circuits without RCD protection.
I'm sure 30mA RCDs get fitted in series all the time but I'm also sure the wiring regs don't condone that.

 

[JohnW2]

Overload and fault protection is primarily there to protect against fire.

Overload protection certainly is. However, if an L-E fault creates a fire risk, it is because of excess current flow (through the L as well as anything else), so overload protection ('alone') is again surely enough to give the 'protection against fire', just as it would if the excessive current were flowing through N as well as L - after all, an OPD does not know where an excessive L current has 'gone'.

Same goes for 100mA RCDs on TT systems,

Again, fire risk results from excessive current flow (through L as well as anything else), so (assuming the fault is in a final circuit) will be addressed by OPDs, in the same way that overload currents are addressed. The only additional protection (against fire) that an up-front 100mA RCD would seem to provide is in relation to L-E faults between that RCD and the devices in the CU - but that is an incredibly improbably occurrence (and with TN installations, there is no protection against such incredibly rare events other than from the cutout fuse).

it's not there primarily to protect against electric shock ...

One can argue about 'what it is there for', but I would say that it does potentially provide considerably more protection against electric shocks than does OPD-based ADS.

As people here are very aware, I am far from convinced that many lives have been saved by RCDs operating as a result of people receiving electrical shocks, but (as I wrote) they do have the ability to detect and clear a fault which was resulting in an exposed-c-p becoming dangerously live that OPD-based ADS would not do anything about - so will clear faults that the OPD could not have cleared-before anyone has a chance to receive a shock from the live exposed-c-p. OPD-based ADS only works if the fault is close to being 'of negligible impedance', which is far from necessarily the case with potentially lethal L-E faults.

and is not there for redundancy, it's ensuring ADS in a L-E fault on the supply cable/circuits without RCD protection.

If you're talking about a TT installation with some final circuits which are not otherwise RCD-protected, then I agree.

I'm sure 30mA RCDs get fitted in series all the time but I'm also sure the wiring regs don't condone that.

As I said, I would personally never knock redundancy of (any) safety measures. I've observed that we often see people being discouraged from having RCDs ('unnecessarily') in series, and rarely, if ever, see people being encouraged to do it - so, for betteror worse, that is consistent with what you say.

As far as I can see, the only significant downside of RCDs in series is a little (probably 'once in a blue moon') 'inconvenience', which I would personally regard as a reasonable price to pay, particularly given that we are led to believe that 'failure' (or, at least, off-spec performance) is not all that rare in in-service RCDs.

 

[trojanhawrs]

Overload protection certainly is. However, if an L-E fault creates a fire risk, it is because of excess current flow (through the L as well as anything else), so overload protection ('alone') is again surely enough to give the 'protection against fire', just as it would if the excessive current were flowing through N as well as L - after all, an OPD does not know where an excessive L current has 'gone'.

A MCB or fuse is overload and fault protection, other than in a TT scenario where the Zs is too high to ensure disconnection. RCDs in TN systems are classed as additional protection.
Sometimes higher mA RCDs are specced where there is likelihood of mechanical damage (rats etc), current can make its way to earth through anything at that point and be a fire hazard without the cable itself overheating.
A new TT installation today would likely not need a 100ma RCD up front with the inclusion of 30ma protection for lighting circuits in the 18th

As far as I can see, the only significant downside of RCDs in series is a little (probably 'once in a blue moon') 'inconvenience', which I would personally regard as a reasonable price to pay, particularly given that we are led to believe that 'failure' (or, at least, off-spec performance) is not all that rare in in-service RCDs.

And the price. I don't think it's dangerous, just unnecessary and annoying.

 

[JohnW2]

A MCB or fuse is overload and fault protection, other than in a TT scenario where the Zs is too high to ensure disconnection.

Indeed.

RCDs in TN systems are classed as additional protection.

Yes, I realise that's what BS7671 'classes' them as, but that does not alter what I said - namely that they potentially offer much more protection against electric shock than an MCB or fuse ever could.

As I said, I actually do wonder how often OPD-based ADS actually 'works' in the face of real-world (rather than hypothetical 'negligible impedance') L-E faults, whereas an RCD will clear faults due to L-E faults of considerably greater (than 'negligible) impedance (but still potentially dangerous).

A new TT installation today would likely not need a 100ma RCD up front with the inclusion of 30ma protection for lighting circuits in the 18th

That's probably generally true, although not true of my TT installation. All of my final circuits have, for many years been RCD- or RCBO-protected but I nevertheless still need up-front 100 mA RCDs in order to provide fault protection to the lengthy distribution circuits.

And the price. ...

In a field which is moving towards SPDs and AFDDs etc. (the need for which in domestic installations is, to my humble mind, very questionable), I think that the cost of additional RCDs is probably pretty trivial.

... I don't think it's dangerous,....

I don't think that anyone has suggested that having multiple RCDs is 'dangerous'. I don't think there is any conceivable way in which it could be electrically dangerous. Potential 'danger' would only exist if the device(s) were feeding safety-critical loads (e.g. 'life supporting equipment') but that danger would exist even with just one RCD, but would be increased by their being two or more.

just unnecessary and annoying.

As I've implied, I'm not sure that it is ever sensible to say that redundancy of any protective devices/measures is 'unnecessary', in that it will always increase safety, particularly when one is talking about devices/systems which are known to sometimes fail in service without anyone being aware that it has happened. If one feels that having a backup for a device which could fail in service is 'unnecessary' one has to ask how 'necewssary' the first one was.

As for 'annoying', as I wrote, we are talking about (probably 'once-in-a-blue moon') slight inconvenience, which could be regarded as a reasonable price to pay if there were a significant increase in safety. I have experienced so few RCD/RCBO trips in the ~40 years I've been living with them that I would hardly have noticed the difference had two RCDs operated on each occasion.

 

[trojanhawrs]

Indeed.

Yes, I realise that's what BS7671 'classes' them as, but that does not alter what I said - namely that they potentially offer much more protection against electric shock than an MCB or fuse ever could.

Absolutely, but that's not what the fuse is there for. The basic protection against electric shock is enclosures and insulation preventing contact with live conductors. A "fault condition" is basically considered to be a dead short (to N or E or exposed conductive part) and a fuse or MCB protects against that as well as an RCD - and I'd argue less likely to fail due to the simple mechanical designs.

As I said, I actually do wonder how often OPD-based ADS actually 'works' in the face of real-world (rather than hypothetical 'negligible impedance') L-E faults, whereas an RCD will clear faults due to L-E faults of considerably greater (than 'negligible) impedance (but still potentially dangerous).

There's no need for inverted commas, in the real world live to earth faults tend to occur when live comes into contact with earth.

 

[ericmark]

I find as an electrician we tend to jump to conclusions without considering all factors. Specially when we are not on site.

The use of metal cladding and/or doors can change what is required, and selecting where earth rods are installed is not easy.

I am sure where I work the earths do not comply, at least three supplies to a single metal framed building, with PME and an EV charging point. But I am also sure in this case it does not matter having seen the DNO transformers and other earthing arrangements.

I point this out, as it is so easy to make errors when not on site.

I find the idea of a protective device braking the earth connection wrong, but know this is required due to the use of PME in some situations, to my mind PME should have never been allowed, but too late now.

However the main problem with DIY is lack of test equipment, it is simply too expensive for DIY guys to buy, so it comes down to trusting the Lord.

 

[trojanhawrs]

I find as an electrician we tend to jump to conclusions without considering all factors. Specially when we are not on site.

Agreed, its good to have a discussion though - a lot of what's in the regs is open to interpretation and there's usually more than one way to skin a cat. The initial question asked was if a cable would be suitable, but it's rarely that simple.

 

[JohnW2]

Absolutely, but that's not what the fuse is there for.

You appear to be rather contradicting yourself, given that you previously wrote ...

A MCB or fuse is overload and fault protection .....

.. and, in the absence of RCD protection, it is indeed true that the MCB or fuse is the only thing providing any fault protection. As you go on to say, as far as far as BS7671 is concerned, in TN installations RCDs 'only' provide 'additional protection'. To be 'additional', it obviously has to be additional to something else (i.e. the OPD-based ADS).

,The basic protection against electric shock is enclosures and insulation preventing contact with live conductors.

Very true, but it's not Basic Protection which we are talking about.

A "fault condition" is basically considered to be a dead short (to N or E or exposed conductive part) ....

Indeed it is ("a fault of negligible impedance")

and a fuse or MCB protects against that as well as an RCD ...

It does - but (as below) only if it really is a "fault of negligible impedance"

- and I'd argue less likely to fail due to the simple mechanical designs.

You may be right about that - I don't know. However, given that one of the main reasons for malfunction of RCDs is said to be 'stiction', and since I would guess that RCDs tend to operate more often than MCBs (most of the many MCBs in my house have not operated in the past 30+ years), I might expect it to be at least a great problem with MCBs.

There's no need for inverted commas, in the real world live to earth faults tend to occur when live comes into contact with earth.

Yes, but coming into contact with" and "having a negligible impedance path to" are, in the real world, not the same thing. You surely must have looked at a (correctly 'zeroed') resistance meter whilst 'touching together' two conductors. One often has to fiddle about quite a lot to get the reading anywhere near zero.

Don't forget that final circuits often have a Zs very close to the maximum that will guarantee ADS with the required disconnection time (e.g. magnetic tripping of an MCB). That means that the impedance of a fault will sometimes have to be only a small fraction of an Ohm to prevent the desired ADS performance. When a 'fault' (even if not such by BS7671 definition ) develops as a result of, say, deterioration/breakdown of insulation or water ingress, the impedance of the path may be of very appreciable impedance - quite possibly still very dangerous but far to high for effective OPD-based ADS. RCDs are obviously particularly valuable in the case of gradually developing (progressive) 'L-E leakages', which they will "stop in the bud".

 

[ebee]

True we always base OPD short circuit and earth fault protection to be with "bolted faults" (negliable impedance) where we know that real world we might well be faced with lightly touching rusty parts of higher impedance and this RCD action might actually be the device that is saving us from considerable grief.

A 30mA RCD is assumed to theorectically be capable of saving 95% of those who might not be saved otherwise.t
Translate that as a 5% failure to protect and that encourages to be safety conscious in all other respects too.
In my (limited) experience then RCD failures of two types, the most common failure being failure to act within spec
(Perhaps stiction? Being the chief contributor) . The other, rarer type I M H O, is complete failure to operate at all.
Often we are only aware of The most common type because of testing with test gear and ,worryingly, we almost never become aware of any MCB failures outside spec that are not complete failures to operate at all. We take assumptons from type testing alone.. similarly with the humble fuse..