[SOLVED] Current Transformer High Resistance Burden - No Flames?

Hi all,

I'm trying to understand current transformers better. A precaution that one can't avoid when reading about CTs is the open-circuit burden. Everyone says it's very dangerous and I've watched videos of CTs bursting into flames because of it.

I thought I'd test this out in a controlled environment. I wound about 300 turns onto a small core (1.5" O.D. toroidal) I had lying around, stuck a cable through it and ran 150 amps through it. Nothing happened -- no flames, no smoke, no arcing.

I did carefully rub the wire ends against each other and got arcs, however, so I knew something was happening. I put an 10:1 oscilloscope on the terminals and got the theoretical peaky waveform with peak voltages of about 600V.

What is happening here? My theory is that the enamel insulation or a component is breaking down at 600V but there is no smoke because not much power is being transferred by the CT.

Can anyone shed some light on this?

Many thanks.

Current transformers are using high permeability tape wound cores. Did you?

Did you run both wires thru or just one as required ?

FvM said:

Current transformers are using high permeability tape wound cores. Did you?

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Yeah. The core was taken from an existing current transformer.

SunnySkyguy said:

Did you run both wires thru or just one as required ?

Click to expand...

Just one wire. The return wire didn't go through the core.

There are two possible failure modes:
- winding insulation breakdown with flashover
- core overheating due to saturation losses

Your experiment was probably too short to observe the latter case.

Thanks FvM.

Do you know if the flashover would be visible? Also, would it develop into something more dangerous if it was left like this?