Safe Isolation Dilemmas.

[securespark]

1. SELV transformer connected to LV supply cable with insulated crimps. You cannot access the metal inside the crimp connectors.

2. A circuit that is already dead when tested, yet no protective devices have failed.

3. Similar to 2. A buried cable that has been damaged by a nail. The nail is removed. The fuse has been replaced. The fuse stays intact. There is a supply to all sockets bar one.


How do you safely isolate these, following standard, accepted safe isolation procedures?

 

[PrenticeBoyofDerry]

1) Could be done using a clamp meter. But I see the dilemma, I would not trust a voltage stick/pen for safe isolation.
2) Then further investigation is needed, as there is a fault on the circuit and I would then isolate the whole system and do individual circuit dead tests. To identify the circuit, then appropriate test on the circuit identified, to find where the fault lies.
3) Again there is a fault on the circuit and further investigation is needed, If as you have already stated you have removed/replaced a fuse for this circuit.
If the power to this socket is not and was not present, you can not be sure the fault lies at that protective device, so as 2 further investigation is needed.

 

[bernardgreen]

A clamp meter would only indicate if there was current flowing in the wire in the clamp.

The only 95% safe isolation is to remove the supply to the property at the incoming source. 95% as there may be another unkown source to the property. Unlikely in the domestic situation but not uncommon in commercial and industrial premises.

Long time ago a derelict factory was about to be demolished, electrical substation had been removed so there was no power. But one light remained glowing. A war time (?) emergency supply from a nearby factory was the supply to the lamp and some office sockets via an automatic change over unit.


That said domestic properties divided into multiple occupation can have some dangerous cross connections between apparently electrically separated flats.

 

[ban-all-sheds]

1) Is there enough slack in the cable to allow you to cut and reconnect?

2) How do you know you're looking at the protective device, or the DB/CU for it?

3) Might not be similar - the nail could have broken line or neutral and not left a permanent fault, so when removed the fuse can be replaced OK but a socket could still be dead. Repair the damage and see if the socket then works and is on that fuse?

 

[securespark]

But how would you isolate safely first??

 

[PrenticeBoyofDerry]

A clamp meter would only indicate if there was current flowing in the wire in the clamp.

.

I have a clamp meter that detects both amps and volts.

 

[PrenticeBoyofDerry]

But how would you isolate safely first??

In regards to question one you don't unless you use a clamp meter that can detect AC voltage.
I would not consider it a safe method to cut in to a cable that has not been proved dead. Even if I discounted the mains supply, I would still be wary!

 

[westie101]

In all cases isolate every possible circuit in the premises that could be the faulty one, only re-energise unaffected circuits when the faulty one has been made safe and identified

In other words switch them all off at the main switch(es).

We would do the same if we were unsure of a cable, isolate every cable in the vicinity and identify them individually

 

[securespark]

Mmm. I agree with you that a clamp meter would be very handy.

I would not consider it safe to cut into that cable either. And if you could not get a clamp meter round the cable, how would you safely isolate?

I was directing that question mainly to Ban.

 

[PrenticeBoyofDerry]

I was directing that question mainly to Ban.

I guessed that and you have been privileged that he managed to answer a question without quotes and regulations.
He must be ill.
I have a martindale clamp that detects both amps and volts, not sure of model no. But will be in use tomorrow so I will post model number.
But as regard question one, you would have to draw the main fuse , to be relatively sure.
The only other options is functional operational test of equipment, but again I would be wary!

 

[JohnW2]

I have a clamp meter that detects both amps and volts.

It surely can't measure volts via it's clamp - that would make no sense. It may have a no-contact voltage detector built in, but that would not really be any different from a 'volt stick', would it? Whatever, I'm sure I would not trust it to prove that a cable was dead.

Kind Regards, John.

 

[PrenticeBoyofDerry]

I have a clamp meter that detects both amps and volts.

It surely can't measure volts via it's clamp - that would make no sense. It may have a no-contact voltage detector built in, but that would not really be any different from a 'volt stick', would it? Whatever, I'm sure I would not trust it to prove that a cable was dead.

Kind Regards, John.

That's a fair point but it does give a voltage value on display, I'll have it out tomorrow, never let me down yet, unless I have been using it incorrectly and been really lucky, don't rule that out
But I must admit I would 100% prefer to use two probe tester, can't you sneak the probes up in to the crimp terminals.

 

[JohnW2]

That's a fair point but it does give a voltage value on display, ....

That's rather intriguing - does it given even remotely reasonable voltage figures?

But I must admit I would 100% prefer to use two probe tester, can't you sneak the probes up in to the crimp terminals.

I thought that - unless they have been heat-shrinked, I would have thought that one could get a fine test probe down as far as metal - even if one had to adapt ones probe to be fine enough with a pin!

Kind Regards, John

 

[westie101]

I thought that - unless they have been heat-shrinked, I would have thought that one could get a fine test probe down as far as metal - even if one had to adapt ones probe to be fine enough with a pin!

Depending on conductor size the Drummond test lamp (http://www.pat-services.co.uk/martindale-mtl7-drummond-test-lamp-1800.htm?utm_source=froogle&utm_medium=ppc&utm_term=1800&utm_campaign=froogle) with straight probes can pierce either paper or XLPE insulation. These are the ONLY type approved for out use

 

[Electrifying]

That's a fair point but it does give a voltage value on display, ....

That's rather intriguing - does it given even remotely reasonable voltage figures?
Kind Regards, John

Am I missing something here, or have you completely lost the plot, John??

You might be able to measure that a voltage is present, but how the hell would you take an actual reading.
Voltage, or should I say potential difference, as the name implies, is a difference in potential between two points - How's that work with a clamp meter??

What two points is the voltage being measured between prenticeboy - when you get your reading??