Dodgy trade pictures for your amusement! - 1 Million Views!

[Darkwood]

Right ... Just been nudged to set this up by Paul.M and sounds a good idea following recent threads I've done in the Arms..

Rules....No Offensive material... edit if required before posting as this is the public arena.
Anything to do with the trade or in and around it ...H&S pic's welcome.



I've posted this a few times and this is at a mates house following a kitchen refirb several yrs ago. mg_smile:

 

[dksanders]

Again correct, but only with clipped direct. Installed in a mobile home 16A likely for 1.5mm. Anyway, from a design perspective, Reg 433.1.1 (ii) says protective device current setting (In) must be < = the conductor current carrying capacity (Iz). This ensures the device operates before the conductors overheat and damage the cable.

The cable is a 2.5mm T&E with CPC of 1.5mm, but it's installed in insulated stud walls. The single socket had a daisy chain of 2 x 4 extension leads plugged into it. Given the state of the other electrics on the place and the owners blasé attitude to electrical safety, and to put it politely, the background of the one big family living in these caravans, I wouldn't be surprised if a 13A fuse that keeps blowing, wouldn't be bypassed. I always err on the side of safety and do not like to see a spur on anything more than a 16A MCB, especially given this environment and type of use.

The only reason he had the EICRs done (last done in 1999) was because he wants to develop the site and put more caravans on and this was a hoop he had to jump through to get either the funds or approvals. (not sure which as I don't speak foreign gypsy).

BTW I put a 6A in for the lights and a 16A in for the spur and added the costs of the MCBs only.

 

[johnduffell]

The cpc csa of 1mm² may not be large enough to handle the fault current;

hi Richard I did a bit of learning and calculation based on your comments, never got round to learning how to use the adiabatic equation before this.
Seems that 1.5/1mm would only comply down to a Ze of just over 0.2ohms (pefc 1150A) but once you get to 6kA you would need a CPC of 1.85mm2, which seems surprising given a standard ring only has 1.5mm CPCs. Is this another special dispensation of a ring?

 

[Richard Burns]

hi Richard I did a bit of learning and calculation based on your comments, never got round to learning how to use the adiabatic equation before this.
Seems that 1.5/1mm would only comply down to a Ze of just over 0.2ohms (pefc 1150A) but once you get to 6kA you would need a CPC of 1.85mm2, which seems surprising given a standard ring only has 1.5mm CPCs. Is this another special dispensation of a ring?

I was mainly referring to the minimum cpc sizes given in table B7 of the OSG for a class 3 energy limiting device such as an MCB.
For a ring the legs are taking current in parallel so the cpc may be considered to be somewhat similar to a 3mm² conductor.

 

[johnduffell]

I was mainly referring to the minimum cpc sizes given in table B7 of the OSG for a class 3 energy limiting device such as an MCB.
For a ring the legs are taking current in parallel so the cpc may be considered to be somewhat similar to a 3mm² conductor.

Fair enough those minimum sizes are presumably for worst case and to save doing the calculation.
Regarding the 3mm, i see what you're saying but i don't think it applies for these purposes. That equivalent would be only at the centre of the ring, but the worst case for pefc would be a fault close to the origin of the circuit. (As opposed to ccc and vd where it would be at the most remote point)
In fact, in the case of an unfused spur from the origin, there wouldn't be a second leg at all to share the earth fault current.

 

[Richard Burns]

Yes the OSG is the idiots guide to installing without thought but can have some useful data when I am too lazy to calculate.
I would suggest that someone has spent a long time designing cables and applying engineering principles to ensure that the reduced size of the cpc is suitable for the majority of situations (assuming the electrician is paying attention to design).
Generally in domestic installations the fault current is quite low and a 1.5mm² cpc would not be a problem. It would be down to the designer to account for a high fault current situation, which I would not expect to be accounted for in most cases.
It is notable that many other countries do not allow reduced size cpcs so perhaps they are on to something.

 

[Wilko]

Hi - probably JD is using 100ms disconnect time from BS7671 ? IIRC using the manufacturer's detailed let through energy (reflecting shorter actual disconnect times) it worked out ok for 6kA. I haven't checked this recently so it might all be rose tinted glasses .

 

[davesparks]

t


true but only where the OPD is necessary: 433.3.1 (ii) allows the overload protective device to be omitted completely in this case (load not liekly to carry overcurrent)

For a socket outlet overload protection is required.
The ref you are referring to allows overload protection to be omitted where the load is not subject to overload faults, with a socket outlet the load is unknown as anything can be plugged in therefore must be assumed that it could be subject to overload.

 

[johnduffell]

Hi - probably JD is using 100ms disconnect time from BS7671 ? IIRC using the manufacturer's detailed let through energy (reflecting shorter actual disconnect times) it worked out ok for 6kA. I haven't checked this recently so it might all be rose tinted glasses .

Good theory, but that would need much bigger conductors, i used the class 3 energy let through for 32A 6ka which was quoted at 45000A²s if i remember right. That was described as necessary for pscc above 1kA

 

[johnduffell]

with a socket outlet the load is unknown as anything can be plugged in therefore must be assumed that it could be subject to overload.

Fair enough thanks, i did wonder if i could count a socket outlet as a "load".
In that case what allows an unfused spur on a ring final? Is that purely a special dispensation rather than due to the 20A Max for a double socket making an overload unlikely?

 

[Wilko]

Probably get razzed for putting this on the bad instal thread, but not to worry .
Just for fun - the Wylex data sheet seems to give 6kA let through for 32A MCB about 3.5x10^4 and plugging that into 434.5.2 with k=115 gave me about 1.4mm2 (hopefully not made a slip ... ). Although it's allowed, I don't use 1.5/1 in any socket circuit. Unfused spurs on 32A RFC should be 2.5/1.5 (App 15).

 

[johnduffell]

Got it, you're a genius @Wilko the thing I was missing was that the let through energy is also limited by the cutout fuse if the MCB can't operate quick enough. So we're actually limited to less than 20000A2s Makes perfect sense now!

 

[Wilko]

Good, now you'll need to splain it to me ...

 

[johnduffell]

Good, now you'll need to splain it to me ...

haha, well the way I understand it now, which hopefully is fairly accurate:
the let through is limited by any and all upstream OPDs
When a fault occurs, each OPD sees the same current flowing (PSCC or PEFC) and trips according to its time current curve given enough time.
The time-current curve of an MCB and a cutout fuse start off the way round you'd expect at low currents, but at very high currents (>kA) they cross over. This is due to the fact that the fuse melts quicker with a higher current, whereas the MCB has mechanical inertia so the time to unlatch is not sped up much regardless of the current. The arc time is presumably similar for both.
Therefore at these high currents, the cutout fuse will blow before the MCB trips, limiting the I2t to the fuse's I2c, therefore limiting the need for a larger CPC.

Make sense? I hope!

 

[Strima]

Mother in law has just had a summer house added to her garden a hot tub going in soon. SWA supply for the hot tub is ready to go but I'm not sure of the isolation carried out...

The SWA feeding the summer house isn't glanded to earth at any point either...

That's it running past the bottom of the meter...

I just have to wait for the certification to turn up before making any comments to the installer.

 

[MFS Electrical]

Dodgy Dave has been doing the rounds again. These are all in the same place. Where do I begin can fit your finger through every one of the holes on the top surface no protection for the cables entering the bottom or the side dodgy set up where that 2.5 t&e’s nuetral feeds something else in the CU god knows why old heating box tails enter two different holes unused holes in the old heating box that you can stick your finger in random unprotected joint down below CU.
Oh and the MCBs etc haven’t been fitted correctly so there is half a device’s width at both ends of the CU with no blanks aaaand a lovely pokey up bit of uncapped busbar just waiting to zap you if you try and reset your tripped lights in the dark and put your finger through the hole. What a mess...