Archived Forum

Why not read about current transformers and their installation and use in Building Blocks? The answer is in there.

Thanks, I did. The warning at the bottom of the page is great and all but so general - apart from the mention of a zener diode in the supplied clamps - as to be confusing. Not everyone here is a qualified electrician.

Sorry, let me rephrase that more politely.

I meant that there is small bits of info and headline warnings over various pages but I could not find a single statement "The supplied VT clamps are voltage limited through integrated Zener Diodes but as they do not have a burden resistor they are still not safe to leave open circuited and doing so can damage them beyond repair. Don't unplug them while clamped onto a live circuit."

It would be great if it said it that simply for end-users like me.

I meant that there is small bits of info and headline warnings over various pages but I could not find a single statement "The supplied VT clamps are voltage limited through integrated Zener Diodes but as they do not have a burden resistor they are still not safe to leave open circuited and doing so can damage them beyond repair."

Hi Peter,

If a CT has integrated Zener diodes, it will not develop dangerous voltage across the leads (provided the diodes haven't failed) if it's clipped around a current-carrying conductor and the CT leads are not connected.

Don't unplug them while clamped onto a live circuit.

Correct. Always unclip the CT from the current-carrying conductor before disconnecting the CT's leads.

and,

Always connect the CT's leads before clipping the CT around a current-carrying conductor.

Bill

Would something like these warnings and explanations suit you?

"Safety
In general a CT must never be open-circuited once installed. A CT is potentially dangerous if open-circuited.

If open-circuited with current flowing in the primary the transformer secondary will attempt to continue driving current into what is effectively an infinite impedance. This will produce a high and potentially dangerous voltage across the open secondary [1]

Some CT's have built-in protection. This can be either protective zener diodes in the case of the SCT-013-000 that is recommended for use in this project, or if the CT is a voltage-output type, it will have a built in burden resistor and so cannot be open-circuited."

or

"Warnings

A current transformer is a current source. It will therefore attempt to generate whatever voltage is necessary to drive that current into its burden resistor. If the burden resistor is not connected, that voltage might reach dangerous levels which could break down the insulation and destroy the C.T.

The YHDC transformer sold in the Shop incorporates Zener diodes to limit the maximum unburdened voltage output to a safe level. C.T.s with internal burden resistors (‘the voltage output’ type) are also protected against developing dangerous voltages.

Always connect the C.T. leads before clipping it round a current-carrying conductor, and always un-clip the C.T. from the current-carrying conductor before disconnecting its leads.

Never open-circuit a C.T. whilst it is on a current-carrying conductor. It is always safe to short-circuit a C.T.

NEVER attempt to fit a C.T. to a bare conductor unless you are absolutely certain it is designed for that. Apart from the obvious danger of electric shock, there are two factors to be considered: the strength of the C.T's insulation, and its ability to withstand the higher temperatures at which bare conductors normally operate."

Thanks Bill, that's nice, specific, clear and brief.

Robert and/or Bill - 

Since I am not an EE, I want to make sure I understand the theory behind safe handling of CTs. In GENERAL, it is not a good idea to open circuit a CT as you can cause damage to it, however if we are using the SCT-013-000 recommended here, that is not a problem due to the Zener diodes installed in the CT.

Is this a correct assumption?

---snip---

Some CT's have built-in protection. This can be either protective zener diodes in the case of the SCT-013-000 that is recommended for use in this project, or if the CT is a voltage-output type, it will have a built in burden resistor and so cannot be open-circuited.

---snip---