MCB for a compressor

[JohnW2]

With RCD protection there is no need for 55-0-55 or 63-0-63 volt systems

Are you sure of that Eric? Remember the most likely path to earth is via a human body, so someone has to have a potentially fatal shock before there is any chance of an RCD operating. There's also the risk of a muscular reaction causing a fall from height. ... This was reviewed by a BSI panel in 2013 as part of the routine review of BS 4363:1998+A1:2013. Specification for distribution assemblies for reduced low voltage electricity supplies for construction and building sites, and there was general agreement that the need for 55-0-55 V systems should remain. I believe HSE had some statistics that prove UK practice to be safer than the rest of Europe's reliance on RCDs

Indeed - in terms of the risk of electrocution, or other consequences of electric shock, it has surely always got to be the case that the lower the voltage (relative to earth) the better, and that reliance on RCD 'protection' can never be a substitute for that. RCDs can fail and, even when they work, they will not necessarily prevent death in sufficiently susceptible or unlucky people, or necessarily prevent injuries due to falling off ladders etc.

Eric is obviously right in saying that one has to balance these advantages of lower voltage against increased risk of fire due to the higher currents associated with lower voltages, but I doubt that latter risk would generally outweigh the former.

Kind Regards, John

 

[DaftPunk]

I've worked on construction sites for many many years. I've seen a fault on a 110V supply fed from a 10kVA transformer burn it's self clear without the OCPD at the tx ever operating.

I've also accidentally severed a 110V temporary supply cable and lived to tell the tale. I'd much rather receive a minor shock or no shock at all from a 55-0-55V supply than a bad shock from a 230V supply hoping that an RCD will operate before I die.

I am guilty of owning and regularly using a yellow brick supply, but in my opinion the additional safety it offers far outweighs the risk of a primary side OCPD.

On the other point raised, I remember for certain that several 16A commando sockets fed from a 20A OCPD was a standard circuit arrangement in BS7671, but can find no reference to this in the current version or the OSG.

 

[JohnW2]

I've also accidentally severed a 110V temporary supply cable and lived to tell the tale. I'd much rather receive a minor shock or no shock at all from a 55-0-55V supply than a bad shock from a 230V supply hoping that an RCD will operate before I die.

Indeed - my very point.

On the other point raised, I remember for certain that several 16A commando sockets fed from a 20A OCPD was a standard circuit arrangement in BS7671, but can find no reference to this in the current version or the OSG.

Ah, thanks - that may explain why I couldn't find any reference to it in current regs or OSG (after Risteard said the same thing)! Can you recall if there was any stated logic in allowing that 'extra 4A' - multiple 16A sockets with a 16A OPD would obviously be fine, but that extra little bit of 'leeway' seems a little odd (assuming that, like others here, they didn't want to allow multiple 16A sockets with an OPD >20A).

Kind Regards, John

 

[333rocky333]

I thought the blue ones had 16/20 amp stamped on them these days

 

[JohnW2]

I thought the blue ones had 16/20 amp stamped on them these days

Interesting. If that's true, then I suppose it could be 'an explanation' - but exactly what would you interpret a rating of "16/20A" as actually meaning?!

Kind Regards, John

 

[DaftPunk]

I've also accidentally severed a 110V temporary supply cable and lived to tell the tale. I'd much rather receive a minor shock or no shock at all from a 55-0-55V supply than a bad shock from a 230V supply hoping that an RCD will operate before I die.

Indeed - my very point.

On the other point raised, I remember for certain that several 16A commando sockets fed from a 20A OCPD was a standard circuit arrangement in BS7671, but can find no reference to this in the current version or the OSG.

Ah, thanks - that may explain why I couldn't find any reference to it in current regs or OSG (after Risteard said the same thing)! Can you recall if there was any stated logic in allowing that 'extra 4A' - multiple 16A sockets with a 16A OPD would obviously be fine, but that extra little bit of 'leeway' seems a little odd (assuming that, like others here, they didn't want to allow multiple 16A sockets with an OPD >20A).

Kind Regards, John

I'm not sure of the logic behind it, but I guess the thinking was that anything requiring more than 16A would be fitted with a ≥32A plug, so the circuit arrangement would allow several pieces of lower power equipment to be connected to a single circuit. As 16A is the lowest CCC of these type connectors, equipment requiring as low as 0.1A may use them purely for their greater IP rating rather than specifically for their higher CCC rating.

I trained to the 1992 regs. I will have a look through my books and see if I can find the historical reference to 20A supplies for 16A BS 60309 type socket outlets.

Personally I've never seen a plug or socket marked as 16/20A.

 

[JohnW2]

I'm not sure of the logic behind it, but I guess the thinking was that anything requiring more than 16A would be fitted with a ≥32A plug, so the circuit arrangement would allow several pieces of lower power equipment to be connected to a single circuit.As 16A is the lowest CCC of these type connectors, equipment requiring as low as 0.1A may use them purely for their greater IP rating rather than specifically for their higher CCC rating.

Indeed. That's really the basis of what I've been asking/saying about teh concept of allowing multiple 16A sockets on the same circuit. From what I understand, most people are happy with that so long as there is a 16A OPD - but I was wondering why, assuming your and Risteard's recollections are correct, the regs apparently allow (or, at least, did in the past) multiple 16A sockets with a 20A OPD.

I trained to the 1992 regs. I will have a look through my books and see if I can find the historical reference to 20A supplies for 16A BS 60309 type socket outlets.

Thanks.

Personally I've never seen a plug or socket marked as 16/20A.

FWIW, nor have I, but that doesn't prove much! However, as I said, I'm not really sure how I would interpret a rating of "16/20A" if I did see it!

Kind Regards, John

 

[delmel]

not good practise putting the over rated mcb in, now the best protection could be spotting a fire if a problem occurs.