Clamp meters

[ban-all-sheds]

Time to hang my head in shame. Was talking to #1 son recently about clamp meters, and he frowned slightly and said "Inverse square law - how does it know how thick the insulation is?"

I frowned a little more, and thought "B*****ks. Good question. Where is my good answer?"

 

[JohnW2]

Time to hang my head in shame. Was talking to #1 son recently about clamp meters, and he frowned slightly and said "Inverse square law - how does it know how thick the insulation is?"

We're not talking about 'radiation'. The inverse square law is not relevant to a transformer with a '1-turn winding'. If it were, then the position of the conductor within the jaws of the clamp, let alone the thickness of insulation, would be critical.

Kind Regards, John

 

[ban-all-sheds]

What induces a current in the jaws of the clamp?

Even if not inverse-square, is there no relationship between field strength and distance?

 

[JohnW2]

Even if not inverse-square, is there no relationship between field strength and distance?

As I understand it ....

.... Only, I think, in terms of possible losses. That apart, a transformer with primary and secondary on different 'arms' of a (iron, ferrite or whatever) core performs exactly the same as if the two windings are on the same 'arm' (or even overwound). It is the turns ratio which matters, not the proximity of primary and secondary (provided there is a 'magnetic circuit' between them).

Kind Regards, John

 

[ban-all-sheds]

So two coils will behave the same WRT one inducing a current in the other if they are separated by 1mm or 1,000,000 km?

Or there is no difference in the way a clamp meter behaves if you hold it open and leave a big air gap vs how it behaves if the conductor has several 10's of mm of insulation on it?

 

[JohnW2]

So two coils will behave the same WRT one inducing a current in the other if they are separated by 1mm or 1,000,000 km?

Yes, that would be the case if there are no losses provided that they are both on the same core which constitutes a complete magnetic circuit. Creating a core 1,000,000 km wide which had minimal electromagnetic losses might not be too easy!

Or there is no difference in the way a clamp meter behaves if you hold it open and leave a big air gap vs how it behaves if the conductor has several 10's of mm of insulation on it?

No, that is utterly different. If there is even the tiniest of air gaps between the jaws, then the magnetic circuit is incomplete and everything changes dramatically.

Knnd Regards, John

 

[ban-all-sheds]

Yes - sorry - IHNI why I said that - I think a big boy's dog ate my brain then ran away and left it on the bus. Even with what he left behind I know the jaws are closed.

 

[JohnW2]

Yes - sorry - IHNI why I said that - I think a big boy's dog ate my brain then ran away and left it on the bus. Even with what he left behind I know the jaws are closed.

I don't know about you, but I find it a lot more difficult thinking about 'current transformers' than voltage ones. However, if one thinks about the latter, I think it fairly apparent that (losses etc. aside) if you have two windings with a complete magnetic circuit between them (e.g. on the same, continuous, core) then the ratio of secondary voltage to primary voltage is simply equal to the turns ratio, regardless (within reason!) of the proximity of the windings.

Kind Regards, John

 

[ban-all-sheds]

It's the "within reason" that has made me think yeah but no but yeah but what about the fact that the conductor could be touching the jaws or well away from them.

I know it works - it's b****y annoying that I've never wondered why/how.

 

[JohnW2]

It's the "within reason" that has made me think yeah but no but yeah but ...

Again, as I understand it ....

The 'within reason' only relates to losses and 'escape of the magnetic field'. If you could invent a 'lossless core' and contain the entire magnetic field within in, then I think that it really would work if the core were 1,000,000 km wide.

... what about the fact that the conductor could be touching the jaws or well away from them.

The conductor (plus the circuit it is connected to) is a one turn primary within the toroidal secondary winding in the jaws, and, as such, will create the same magnetic field no matter where it is within the toroid. It is the "1-turn" and the current which flows through it which matters, not it's position. Maybe you could think of it in terms of the inductance of that 1-turn 'winding' - the inductance will be the same regardless of its position.

If this didn't work with clamp meters, nor would RCDs operate reliably.

Kind Regards, John

 

[333rocky333]

My clamp meter never worked without the jaws closed, how come the ones on,say a T5 tester, work with the end of the jaw open.

 

[JohnW2]

My clamp meter never worked without the jaws closed, how come the ones on,say a T5 tester, work with the end of the jaw open.

As I've said/implied, I cannot see how a clamp meter can work (even remotely accurately) unless the jaws are tightly closed so that the magnetic path is not interrupted. However, I'm far from an authority on such matters!

Kind Regards, John

 

[flameport]

how come the ones on,say a T5 tester, work with the end of the jaw open.

Hall effect sensors rather than a current transformer arrangement.

 

[JohnW2]

Hall effect sensors rather than a current transformer arrangement.

Ah - fair enough; I was obviously thinking about CT ones. As I was implying (and still believe) I cannot see how a CT-based clamp meter could work (usefully) without the jaws being closed.

Hall effect ones presumably can also work on DC (which CT ones obviously can't)??

Kind Regards, John

 

[333rocky333]

Hall effect ones presumably can also work on DC (which CT ones obviously can't)??

Kind Regards, John

Sorry to butt in on BANS thread
Would they work on 12 volt dc then