Copper tape around Current Transformer

[Al (astral highway)]

Can I screen a pair of current transformers by wrapping a 3/4 turn of copper tape around their outsides (axially) and grounding it?

I can't see how this would affect their performance, since the flux being used to induce a current in their windings is generated by passing a fat wire right through their centres. On the other hand, I don't know if it could influence their behaviour even slightly.

I'm on a learning curve around the art of screening so I thought better to check with the experts... Also, is this an effective RFI measure? If it's pointless, I won't do it.

[Radio Wrangler]

The transformer was invented by a guy called Michael Faraday and he had a bit of a time convincing people that the coupling between windings wasn't simply electrostatic (I.E. capacitance)

So he wrapped a wide copper tape around one winding, allowing the tape to overlap itself, but be insulated from itself so it did not form a shorted turn. In this way his special screen allows magnetic field coupling but prevents electric field coupling. This is a FARADAY SCREEN.

Unfortunately a lot of people have use that name for all forms of screens when it is actually for a special one. Now a lot of people using some word might justify a dictionary entry being changed, but it doesn't turn electric fields into magnetic ones.

Anyway, you can screen one winding from another one. You don't need to limit the screen to threequarters of a turn, you can have a whole one and some overlap, just so long as opposite ends are insulated from each other.

This technique is limited by the resistance/inductance of the ground connection of the screen. The way to get better performance is to have two screens, each insulated from the other, each with a separate connection to ground. and so on. You may come across very high quality mains transformers described as 'Double screened'

David

[G6Tanuki]

You can also screen the winding of a current-transformer by making the primary or the secondary out of screened cable [coax?] but only earthing *one* end of the screen.

I've seen this done on RF current-transformers in antenna-tuners, where you only want to measure the current-into-the-antenna and stray capacitively-coupled voltages can really mess up the balance of the 'bridge'.

I have seen some transformers with an overall copper wrap, joined at the ends, I have no idea what that does though. Could be what you want once someone else has explained it.

[G8HQP Dave]

An overall copper wrap is probably a belly-band. It reduces stray magnetic fields from a transformer, so reducing induction into nearby circuits. Generally only seen on high-quality equipment, or stuff built with surplus transformers.

[Radio Wrangler]

A belly band goes around the transformer windings (all of them) and the core as well.

With the ends soldered together it acts as a shorted turn around the whole transformer reflecting any flux leakage back inwards. It doesn't need grounding to work.

It gives no screening effect from any winding to any other (so you still want waraday screens inside) but it reduces field from the transformer getting picked up in sensitive circuitry.

David

Ah, now I know, thank you!

[Al (astral highway)]

Originally Posted by Radio Wrangler

A belly band goes around the transformer windings (all of them) and the core as well.
With the ends soldered together it acts as a shorted turn around the whole transformer reflecting any flux leakage back inwards. It doesn't need grounding to work.

Perfect! Thanks David, that will do the trick

[Al (astral highway)]

Originally Posted by G6Tanuki

You can also screen the winding of a current-transformer by making the primary or the secondary out of screened cable [coax?] but only earthing *one* end of the screen.

Neat idea, thank you. I'd toyed with it for a moment but... is coax inner-outer to grounded sheath good to stand-off 2-3 kV (pulsed, duty cycle 2-3% probably)?

Has to be absolutely bomb-proof or could cause an expensive headache or two if it breaks down. Oh and what's the stray capacitance approx? how do I work that out?