Spur off MCB?

[JohnW2]

Hi John, Ah sorry, I didn't realise the impedance was measured out to the substation. How on earth do you measure that?!

You don't have to 'do' anything special. When you measure the Zs from any point in a ciruit (like at it's end), you are, by definition, measuring the impedance of the entire loop (through which fault current flows), all the way back to the substation.

I think I will just put another B40 in the garage to protect this new circuit,

That may work, but I wouldn't personally put too much money on that never tripping at compressor start-up. Whatever the compressor's documentation may say about the inrush current, it could probably be anything up to about 10 times the running current - a C32 would be more likely to tolerate that than a B40

I think previously I thought 6mm cable was only rated for 30-something amps, just realised that clipped direct it's 46a so a B40 would do the job fine and I don't need to worry about Zs etc?

Well, as before, you can't even be certain, without measuring it, that the Zs would be low enough for a B40, let alone a C32.

I guess the use of the B40 on the new circuit would mean that the B40 upstream in the house could trip first in the event of an overload but I suppose that's fine?

Yep. The same would be true if you had a C32 in the garage. If you had tripping problems, the solution would, I suppose, be to have a C40 in the house (Zs permitting).

Kind Regards, John

 

[JohnW2]

Where has he said about replacing the board?

As it happens I think I've decided just for peace of mind to just put in a new consumer unit, with 3 MCB slots... then I'll just put the compressor on it's own MCB... for the sake of half an hour to rewire the CU.

Kind Regards, John

 

[sparkwright]

So he has.

Late starter to this topic, can't read all that rubbish!!

 

[securespark]

You have only a 40A RCD protecting circuits in the house CU?

How many is it protecting?

 

[loply]

There's two RCDs in the house, I think it's one for upstairs circuits and one for downstairs circuits... 4 MCBs each I think.

Hopefully the last question - is it worth putting another 30ma RCD in the garage CU? This would be downstream of the one in the house CU, but does it offer additional protection should that one fail to operate (at the expense of having to reset them both if something causes an earth fault)?

I'm always conscious of zapping myself when I'm doing something in the garage so the more protection the better really...

 

[EFLImpudence]

This would be downstream of the one in the house CU, but does it offer additional protection should that one fail to operate (at the expense of having to reset them both if something causes an earth fault)?

I'm always conscious of zapping myself when I'm doing something in the garage so the more protection the better really...

You are correct.

 

[JohnW2]

Hopefully the last question - is it worth putting another 30ma RCD in the garage CU? This would be downstream of the one in the house CU, but does it offer additional protection should that one fail to operate (at the expense of having to reset them both if something causes an earth fault)?

As EFLI has said, that's essentially true - although you might not have to re-set both. Although both might operate in the event of an earth fault, it's not uncommon for just one (either) of them to operate.

You'll see quite a lot of people criticising 'unnecessary' use of two (or more!) RCDs in series, and it is true that you could end up with the inconvenience of having to reset more than one. However, given that faulty RCDs are far from unknown, I don't think one can really knock the concept of redundancy, if that's what someone wants. After all, I expect any car to have a dual braking system these days!

Kind Regards, John

 

[plugwash]

After all, I expect any car to have a dual braking system these days!

On the other hand at least in a TN installation RCDs are essentially the third line of defense against electric shock. The first line of defense is basic insulation. The second line of defense is either EEBADS (with MCBs providing the ADS part) or Double/Reinforced insulation.

 

[Robin0577]

Final question I didn't think of previously - I can't seem to find anywhere that sells flex cable sufficient to go between the compressor and it's 32A plug. Should I just use solid core here? If I do that I might as well just get rid of the plug and wire it straight in to an isolator switch...?

No, if you're using a plug do not use "solid" cable. Use proper H07-RN suitable for the job, it's easily available.

https://www.essentialsupplies.co.uk/acatalog/4mm-HO7-Rubber-3-Core-Cable--EC36_H.html
http://www.stage-electrics.co.uk/shop/sales/cable/power/rubber-h07/product.aspx?code=605-4072
http://www.10outof10.co.uk/acatalog/Cable.html
https://www.arenasoundandlight.co.uk/4mm-h07-3-core-rubber-cable.html

 

[JohnW2]

On the other hand at least in a TN installation RCDs are essentially the third line of defense against electric shock. The first line of defense is basic insulation. The second line of defense is either EEBADS (with MCBs providing the ADS part) or Double/Reinforced insulation.

The basic insulation should prevent there being any touchable 'live parts', albeit only 'in normal use'. EEBADS (or DI) should prevent any touchable parts being (or becoming) 'live', due to a single 'negligible impedance' fault. Together, those two things therefore do provide 'two lines of defence' against electric shock in many situations.

However, there are other situations in which neither of those 'lines of defence' are operative so that, in reality, an RCD (if present) becomes the first (and probably only) defence against electric shock - or, at least, against the consequences of electric shock. In the context of this forum, the most obvious example would be the situation in which potentially live parts are deliberately exposed and someone is working on them (maybe not realising that the parts are live). In that situation, the only 'fault current' could be the current flowing through the person - and nothing other than an RCD could bring that to a very rapid halt. Although I've been much more sensible (and have hence not had any shocks) for many years, in my less-sensible-youth, I think that all the shocks I experienced came into that category.

I have also seen so-called 'reinforced insulation' broken to the extent that live parts become touchable, again creating a hazard that only an RCD can usually do anything about.

Furthermore L-E faults which are not quite 'of negligible impedance' can result in a situation in which (even in a properly-designed installation) 'touch voltage' can be dangerously high, but disconnection by EEBADS can be sufficiently slow for shocks to persist for plenty long enough to be potentially fatal.

I therefore think it's probably wrong to believe that RCDs are always 'the third line of defence' - as above, in some situations they can be the first, and only, line.

Kind Regards, John