clampmeter

[johnmf]

got a clampmeter and want to see how much load a 3 phase db is pulling at the place i work.

can i just hook the clampmeter around the individual phases to measure/ and or all 3 at once. its a decent meter with a good range.

any other safety issues with using clampmeters that i should be worried about?

 

[JohnD]

One at a time. If you put it round all 3 they will tend to cancel each other out. If you include the neutral as well then (apart from earth leakages which might be a worry) they will completely cancel out.

 

[Spark123]

With a bog standard clamp meter clamping around all 3 phases you will get the resultant sum of all 3 phases which in an ideal situation will be zero, (i.e. how a 3phase RCD works).
Some of the better clamp meters can measure like this, some clamp meters can give you things like PF, watts, VA, cos 0.
Without info on your type of clamp meter I would say do the individual phases.

 

[Spark123]

DOH

(Makes note, must type faster)

 

[JohnD]

tee hee

 

[johnmf]

cheers,

is it also allowable to clamp the individul phases of the mains tails coming into the installation? so its possible to see how much each phase is dragging?

is there any danger in doing this?

 

[JohnD]

yes

no

Unless you touch anything live, e.g. poor terminations, damaged insulation, live metal enclosures

You can feel the main earth cables too, hoping to have nothing much.

 

[Lectrician]

Clamping all three tails at once will give you the neutral current

 

[plugwash]

actually it will give you the magnitude of the phasor addition of the neutral and earth currents.

 

[breezer]

if it helps, when i was an apprentice i could never figure out how on a controls panel say, that the meter would read 415v but currrent would be say 140A.

I was wondering how so much current could run through a tiny meter.

My electrician at the time opend the panel and showed me, its got torodial transfromers around each incoming core, the meter actualy measures the voltage induced into the transformer.

so my point is that your clamp meter is like a torodial transformer that you can open to put round a cable to see what current is being drawn (then close it) but you actualy do not interfere with the main circuit, and you find out what you want to know

 

[Lectrician]

actually it will give you the magnitude of the phasor addition of the neutral and earth currents.

Yea - so basically, what you would expect as the neutral current

 

[Spark123]

I think you are talking about current transformers are you breezer? We have some of these on selector switches on the incomers to motor control centres. These convert the high current in the primary to low current on the secondary. In doing this they convert the voltage on the primary up to what can be a scary voltage. Current transformers should never be left open circuit, they should always be connected to a low impedance ammeter or a short circuit put across them. For this reason, fuses are not used in CT circuits.