for trainees

telectrix · Mar 25, 2016

a small exercise for trainees. something to get the brains kicked in on a bank holiday weekend.

hypothetical scenario.....

i have a DB supplying a flat. max. demand is 60A. it is to be supplied from an external meter enclosed in an approved DNO cabinet. the distribution cable length is 30m and the Ze at the meter is 0.18ohms. PME earthing. The meter is a new one where the outgoing terminals can be isolated without breaking seals. The first 20m of the cable is external, with the second 10m clipped direct ( surface above false ceiling ) internally.

1. What type and size of cable?

2. What needs to be fitted in the meter box?

3. What essential tests need to be done ( apart from the final circuits, can be assumed that all have previously been tested OK).

hightower · Mar 26, 2016

Pfft I'm not doing your homework for you!

When you say external would that be buried? Will have a crack when I get home

telectrix · Mar 26, 2016

external as in fixed to the brickwork on the outside of the building. no containment.

hightower · Mar 27, 2016

Okay Tel, I've had a look at this. Bearing in mind I've not done any real circuit design before, please go lightly on me - I'll await your advice with open ears.

1) I would say a 10mm SWA cable - but I'm not sure whether a 2 core or 3 core would be best. I've done the calculations and I think the armour of the SWA could be used as the earthing conductor - however, I'm not certain I've done these correctly and I'm not sure if there's any regulations that don't permit this. I've checked the calculations on the cable size and 10mm meets the CCC and also volt drop requirements. If using a dedicated core for earthing arrangements, it would give a Zdb at the load end of 0.25.

2) I'm really not sure on this, but I would say a 60a breaker in order to protect the cable in fault conditions. My reasoning behind this is the Cutout might have an 80 or 100a fuse which in an overload or fault might allow too much current to flow in the cable before the fuse can disconnect it. I'd also considered whether a time delayed RCD would be required, but I'm not sure it would be.

3) insulation resistance, R1+R2 (and R1+Rn), polarity, prospective fault current, and Zdb.

could be a million miles away with this, but very interested to see how I did being a complete n00b to this. Thanks

Simonslimline · Mar 27, 2016

hightower said:
Okay Tel, I've had a look at this. Bearing in mind I've not done any real circuit design before, please go lightly on me - I'll await your advice with open ears.

1) I would say a 10mm SWA cable - but I'm not sure whether a 2 core or 3 core would be best. I've done the calculations and I think the armour of the SWA could be used as the earthing conductor - however, I'm not certain I've done these correctly and I'm not sure if there's any regulations that don't permit this. I've checked the calculations on the cable size and 10mm meets the CCC and also volt drop requirements. If using a dedicated core for earthing arrangements, it would give a Zdb at the load end of 0.25.

2) I'm really not sure on this, but I would say a 60a breaker in order to protect the cable in fault conditions. My reasoning behind this is the Cutout might have an 80 or 100a fuse which in an overload or fault might allow too much current to flow in the cable before the fuse can disconnect it. I'd also considered whether a time delayed RCD would be required, but I'm not sure it would be.

3) insulation resistance, R1+R2 (and R1+Rn), polarity, prospective fault current, and Zdb.

could be a million miles away with this, but very interested to see how I did being a complete n00b to this. Thanks

A 10mm2 SWA would not comply on Volt drop mate. Remember that there will be Lighting off that sub-main. 6.9volts is your max. I got 16mm2 as compliant.

It would be best to protect the SWA with a switch fuse so as to achieve discrimination with the MCB's. The DNO would require this also.

Is there any extraneous conductive parts in the flat?

hightower · Mar 27, 2016

Yeah tel pm'ed me about the switch fuse. I hadn't realised that about volt drop but just done a little research and of course it makes sense that the drop is from the origin to the final circuits.

telectrix · Mar 27, 2016

1omm cable 4.4mV/A/m.

( 4.4 x 60 x 30 ) /1000 = 7.92V. so at max demand the VD would exceed 3% as you rightly say. however, in the real world, the likelyhood of the installation drawing the full 60A max. demand is rare. it was a ---- up between 10mm and 16mm and in the end 10mm was selected.

Simonslimline · Mar 27, 2016

telectrix said:
1omm cable 4.4mV/A/m.

( 4.4 x 60 x 30 ) /1000 = 7.92V. so at max demand the VD would exceed 3% as you rightly say. however, in the real world, the likelyhood of the installation drawing the full 60A max. demand is rare. it was a ---- up between 10mm and 16mm and in the end 10mm was selected.

The voltage will more realistically be nearer 250 than 230. It is where i live anyway. Obviously this fluctuates.