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DNO Isolation of supply cable.
Former Community Member
After watching a YouTube video in which the DNO was moving the customers service head and they had to join the new cable to the old lead sheathed supply cable, I was wondering how the DNO isolates the old lead sheathed supply cable so they can connect the new cable to it?
I think that the short answer is that they don't isolate - the jointing is done live.
For a customer's recent service head move (from a site cabinet to the outbuilding which was built around it) the quote was for cutting the service cable and (having fitted a new service head) splicing it. In the event, it was possible to swing the service head around to its new position without cutting anything.
As far as I can gather, the DNOs will make a joint in the cable in preference to attaching a service head live.
If I am wrong, doubtless one of the DNOs in here will say so. ?
Unless there are complicating factors, most types of LV cables can be live jointed if access is good and conditions are clean and dry.
As well as the PPE, there are special tools, including shaped insulating wedges to drive between the cores before you hacksaw into them, so that the saw does not short out adjacent cores. Neutrals/ earths are linked to new before cutting any of the old armour, to avoid an open circuit neutral at all costs, and then lives are done joined last. Whatever is not being worked on is covered in temporary insulation.
Might I ask for a linky to said video? I love watching DNO jointers at work... the calmness and skill is amazing. And the PPE is expensive and wonderful to behold.
On the prickly subject of working live, assuming a risk assessment has deemed it necessary, what PPE do other members adopt?
I have 1kv gloves, protectors, a face shield, and an ex fire brigade jacket. Along with ear defenders etc if it's in a confined space near the service head. Obviously I don't bother with anything more than the face shield and gloves/protectors if working on a relatively low fault capacity circuit.
The DNO CAN isolate the supply at the substation, but that interrupts the supply to perhaps a few dozen other customers. Jointing is normally done whilst live as has already been said.
Consumers installations should normally be made dead before work, but if live jointing is needed, then I normally suggest that this be sub-contracted to the DNO or to whatever contractor the DNO use.
On the prickly subject of working live, assuming a risk assessment has deemed it necessary, what PPE do other members adopt?
Not quite answering the question, but I don't work live. R14 of EWR 1989.
I suppose that it is reasonable for jointers to work live given their methods and the inconvenience of cutting off a whole street or two.
When I have seen DNO personnel working, PPE has been conspicuous by its absence. The only exception was a meter fitter, but he was having his 6-monthly assessment.
Many years ago I got Yorkshire Electricity Board (as then was) to move the service head at Wombat Mansions. This involved digging up the PILC cable in the front garden and cutting and splicing it. When I enquired to the foreman how they managed to safely do that live, he replied "Well Mr Wombat, officially our procedures are such that it can always be done safely without any risk of injury. Unofficially ... it makes a hell of a bang."
