Overloaded Supply

[Graham3232]

Hi folks,

I’m looking for some of your experiences with excess loading and services fuses here.

I have set up a temporary site supply for a large office site using an existing supply from what used to be a house on part of the site (now all one building)

The supply was installed in the 1930’s and is a paper/lead cable which had a cast iron cut-out but the DNO have replaced it with a modern plastic one last month as the meter fitter refused to connect the iron one which only had a 40A ceramic fuse holder.

The new cut-out has a 100A fuse and existing 16mm meter tails with new meter to Henley blocks

I set it the site up with electric shower, cooker and sockets/lights. No problems. Domestic type setup.

Problem now is that security staff are now living on site (I wasn’t informed of this happening). They have about 8 convector heaters and have installed another 3kw water heater plus washing machine, dishwasher, dryer etc. and loads of floodlights.

When I called in to check round there was a report of a ‘hot’ smell’ around the main supply point. The service cut-out, tails and meter were all too hot to touch and there was oil starting to drip from the main cable.

I put my clamp meter on the tails and the load was 183A! After switching off some heaters and appliances I got the load to 105A and it has been running between 70-120A for a few days now, oil has stopped leaking!

The new supply is getting connected in 2 weeks so I’m not too concerned long term, but how did the 100A service fuse not fail at such a high load? What load would an old supply cable maybe equivalent of 10 or 16mm take before the cable failed?

 

[JohnW2]

... the load was 183A! ... but how did the 100A service fuse not fail at such a high load?

Were the very high loads continuous? I think a 100A fuse would allow 183A to flow for half an hour or so, and 150A to flow for much longer.

What load would an old supply cable maybe equivalent of 10 or 16mm take before the cable failed?

I can't speak for 'old' cables, but I would personally doubt that, other than getting pretty hot, modern cables of that size would come to any great harm - the 'in free air' ratings of 10mm² and 16mm² are 70A and 94A respectively at 70°C - so we are only talking about roughly double those figures. I suppose the insulation might possibly melt, but I doubt that the conductor would come to any great harm.

Kind Regards, John

 

[kai]

I once encountered SIX flats, each on a 60amp fuse, running off ONE old incomer in Central London, the Incomer ran hot, as it was too hot to touch, eventually the supply cable failed open circuit, and was subsequently replaced with a three phase supply, with a ryefield box to feed the six flats.

 

[stillp]

What protective devices have you installed? You don't mention what's after the Henley blocks.