Underground cable advice needed

[grumbas]

We're having a bit of work done on our office, part of which involves running a new 100A 3 phase supply out to a new building.

The electrician doing the work has only run a 4 core cable and seems to be relying on the armour for the earth which I've been told by someone else isn't allowed but wasnt able to point me to anything in the regs to confirm this - any sparks able to comment?

Also, I was under the impression power couldnt be run with other services yet the cable has been put in next to the water main with zero seperation?

Finally, the last couple of meters of the install the cable has been run literally 25-50mm off the outside wall of the existing building and no more than 200mm down (admittedly under concrete), again this doesn't sound like best practice?

Is there are anyone on here familiar with the latest regs that can give me some guidance on this? As works only just started I'd like to challenge the electrician sooner rather than later if the works not up to standard!

[jeebus]

Sounds ok to me, it is perfectly acceptable to utilize the armouring as the earth, although sometimes you do need to run a seperate earth but only on larger cable's I cannot remember what size off the top of my head, but for an 100A supply you will be nowhere near that size.

[turbospud]

it depends on the size and distance of the cable being used,eg,a 25mm 4core will not need a seperate earth untill its in the 60m range,there is a lot of other factors that can affect it,i like to run seperate earth just in case.same trench is ok but not lying on top of each other

[grumbas]

turbospud said:

it depends on the size and distance of the cable being used,eg,a 25mm 4core will not need a seperate earth untill its in the 60m range,there is a lot of other factors that can affect it,i like to run seperate earth just in case.same trench is ok but not lying on top of each other

It doesn't look much bigger than 10mm and is running around 20-25m.

I always thought there had to be around 200mm seperation in the trench but it's pretty much touching the water pipe most of way?

[turbospud]

for a 100a three phase supply it should be 25mm csa 4core,seperate earth isnt needed at that distance,16mm csa might make it on current carrying capacity but earth resistance might be short

grumbas said:

turbospud said:

it depends on the size and distance of the cable being used,eg,a 25mm 4core will not need a seperate earth untill its in the 60m range,there is a lot of other factors that can affect it,i like to run seperate earth just in case.same trench is ok but not lying on top of each other

It doesn't look much bigger than 10mm and is running around 20-25m.

I always thought there had to be around 200mm seperation in the trench but it's pretty much touching the water pipe most of way?

[Smiler.]

A 16mm² or 25mm² buried cable is not big enough for 100A 3-phase, 35mm² is the minimum size.

The need for a separate earth is dependant upon the overall installation.

Separation between services should be maintained to prevent inadvertent damage should maintenance/repair to another service be required.

The recording of routes/cable sizes on drawings (to scale if possible) is almost always essential in the long term.