clamp meter for measuring ac volts

[amateurSparky]

Hi
On my course we had it drilled into our heads that mutlimeters are bad and we should use one of these
http://www.pat-services.co.uk/martindale-vi13700g-safety-voltage-tester-1864.htm.

If say you found a junction box/cable randomly undernearth the floor. The most hands free approach to ensure it's dead would be a clamp meter right? Is there anything wrong with using one for this purpose?

Would it be safer than a multimeter since you're never in contact with the circuit. IT uses the emi the cable gives off to determine it's voltage.

 

[ricicle]

Clamp meters are mainly used for measuring current (although most of them do have the ability to measure voltage with the correct leads)

Most of them cannot read current anyway in a multicore cable

Current would have to be flowing in the cable anyway for it to indicate that the cable was 'in use'. ie a cable can still be live even if there is no current using equipment on the circuit.

Two probe voltage testers are the only way to prove dead (with the appropriate cross checking)

As for testing a cable only, you would need to trace it to a point where it could be tested with the voltage tester. Unless you can isolate the whole installation and be able to splice into it to test but even then there is no guarantee that the cable is dead.

 

[plugwash]

Hi
On my course we had it drilled into our heads that mutlimeters are bad

I wouldn't say multimeters are bad in general but using them to test for dead is not ideal as their use is more error-prone than dedicated testers (it's still MUCH better than not testing for dead at all thought). Also when using a multimeter for work on mains installations you REALLY should get a set of approved fused test leads to provide protection in the case of internal faults or misconnections.

and we should use one of these
http://www.pat-services.co.uk/martindale-vi13700g-safety-voltage-tester-1864.htm.[/QUOTE]
A good idea.

If say you found a junction box/cable randomly undernearth the floor. The most hands free approach to ensure it's dead would be a clamp meter right? Is there anything wrong with using one for this purpose?

This question seems to demonstrate a STRIKING lack of understanding of what a clamp meter does.

Would it be safer than a multimeter since you're never in contact with the circuit. IT uses the emi the cable gives off to determine it's voltage.

NO

it uses the magnetic field to determine CURRENT FLOW. There can be voltage present even when there is no current flowing. Furthermore most clamp meters can't isolate the magnetic fields from individual cores in a multicore cable so they really need to be used on inidividual cores.

While most clamp meters have a voltage measurement range it does not use the clamp, it uses a set of probes like a multimeter does. It's less risky than most multimeters because there is no risk of misconnection but still it's much easier to misread than a proper voltage indicator.

"volt sticks" can detect voltage on a cable without cutting it but they are not particulally reliable and wont work at all on cables with an earthed metal layer between the live cores and the sheath.

Further even if you have proved something is dead you have no gaurantee it will stay dead. This is especially true if it was already dead when you arrived.

A good general safety rule is to wherever possible make sure at least two different things have to go wrong before you get injured. So even after following "safe isolation" procedures you should still wherever possible treat the wiring as if it was live.

 

[EFLImpudence]

If you found a junction box and are not going to open it what difference does it make and what would it prove whether the cables are live or not?