Minimum protective conductor size

[Tuttle]

CU replacement ... existing ring mains (on 30A 3036 fuses) are wired in 2.5mm T&E with a 1.0mm cpc :sad_smile:

I'm looking at Table B7 in the OSG which seems to say to me that the minimum protective conductor size is 1.5mm for a fault level up to 3kA on a class 3, 32A breaker.

I've got a max prospective fault current of 1.04kA at the origin, which is a fair amount less than 3kA.

How do I tell if this 2.5mm/1.0mm cable is ok on a 32A Type B RCBO?

I tried to use the adiabatic but couldn't work out what the disconnection time / let-through energy of the RCBO would be.

 

[telectrix]

measure your PEFC (or calculate from Zs) at the furthest socket. from that use time/curreng graphs to determine the disconnection time of the MCB. then use that time in the adiabatic.

 

[SirKit Breaker]

You are looking a it too deep mate. Yes you can do the adiabatic equation, but stick to standardisation. Your 2.5mm/1.0mm is fine. It will withstand the fault current up until disconnection. With a PEFC of 1.04kA, you disconnection will be almost instant.

Cheers..........Howard

 

[Tuttle]

measure your PEFC (or calculate from Zs) at the furthest socket. from that use time/curreng graphs to determine the disconnection time of the MCB. then use that time in the adiabatic.

Ok, so I've got Zs = 0.66 at socket outlets, which gives me a PEFC of 348A.

Looking at the characteristic of a 32A BS EN 61009 RCBO in the regs (Fig 3A4), the curve turns into a vertical line at 160A and there is a note that says refer to the manufacturer's let-through energy data for prospective fault currents in excess of those providing instantaneous operation, but I haven't found the manufactures info yet.

I realise that this is probably ok, and certainly better than it was on a 3036, but I would like to be able to justify why it is ok.

 

[MDJ]

Ok, so I've got Zs = 0.66 at socket outlets, which gives me a PEFC of 348A.

Looking at the characteristic of a 32A BS EN 61009 RCBO in the regs (Fig 3A4), the curve turns into a vertical line at 160A and there is a note that says refer to the manufacturer's let-through energy data for prospective fault currents in excess of those providing instantaneous operation, but I haven't found the manufactures info yet.

I realise that this is probably ok, and certainly better than it was on a 3036, but I would like to be able to justify why it is ok.

348? Sure?

 

[Richard Burns]

If you undertake the adiabatic calculation using 348A and a disconnection time of 0.1sec (it will be quicker than this in reality so the resultant csa will be high)
you get a csa of 0.95mm2 therefore this cable will be OK.

 

[Tuttle]

348? Sure?

You obviously think I'm wrong, so no I'm not sure :wink_smile:

I'm thinking 230/0.66 = 348A

Is that not PEFC at the socket?

 

[telectrix]

i get 348A as well.

 

[Tuttle]

i get 348A as well.

He's obviously just trying to worry me, and it worked

 

[MDJ]

You obviously think I'm wrong, so no I'm not sure :wink_smile:

I'm thinking 230/0.66 = 348A

Is that not PEFC at the socket?

no not at all, just checking, it sounded right but I couldn't remember the formula and was hoping you would confirm that was all, nothing else really, apologies if you thought I was trying to confuse you.

 

[Tuttle]

I found some info on Hager breakers, which is too long to read on a Fri night ...

http://www.google.co.uk/url?sa=t&rc...kc8iNi_FGSyO_TQ&bvm=bv.43828540,d.d2k&cad=rja

From a quick search of this doc, Table 34 seems to say: MCB total let-through energy for a 32A MCB and a PSCC of 3kA is is 11kA[SUP]2[/SUP]S

So I'm thinking that I can plug this straight into the adiabatic equation as I[SUP]2[/SUP]t, which gives S = 0.91mm[SUP]2
[/SUP]
So I think that I'm alright which ever way I look at it, unless someone can tell me differently

 

[Doug]

I always thought that 1mm CPC on old 2.5 T&E was only a problem with rewirable fuses (BS3036). The max Zs tables in the OSG show that this is acceptable for up to 20A but not permitted for 30A rewirables. I was taught that a quick solution is a plug-in breaker.

As the OP points out, the Green OSG has a table (B7, page 120) which states that with BS60898 or 61009 devices, 1mm CPC is only acceptable for up to 16A, type B, with max 3kA fault current. For over 16A or a type C, the min CPC is 1.5mm. With fault current between 3 & 6 kA, min CPC is 2.5mm. Some figures given in this appendix are subject to the adiabatic, but not these.

This table was not in the original Red OSG but was on the Errata sheet issued by the IET in 2008.

 

[Tuttle]

Another factor to consider is that this is a ring, so there are two 1.0mm cpcs. I assume that the table in the OSG is for radials so assumes a single 1.5mm cpc. Not sure how much this helps though in a fault situation, especially if the fault is near to the origin on one leg of the ring.