Current Transformer: Safe isolation procedure is always Short-then-Open

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My advice is unwavering for any CT, regardless of application/situation:  "first short the CT-side link, then second open the device-side link".

In the case of circulating current "Merz-Price" High Impedance Differential Bus Bar Protection, I understand and accept that short-then-open renders the BBP scheme inoperative when the short is applied and then it will return to service when the links are opened in the second step. 
Some may perceive an inoperative BBP scheme is a risk to grid security/reliability.
During those few seconds between shorting the CT and then opening the links, you would rely on the duplicate or back up protection to clear any fault that occurred.  That is fundamental protection principles of two systems able to clear any fault independently of the other.

However, I recently came across some comments where it was stated that a common procedure, at least in certain organisations in Australia, and has been for decades, was "first open the relay-side link and then second short the CT-side link" with many a protection engineer somehow being able to explain why it is OK to do it that way (I haven't seen what constitute those explanations).

(On the other hand I was told that for other than HiZ BBP, the procedure is most definitely short-then-open)

I accept that open-then-short is fine IF there is no current flow in the primary - i.e. typically the technicians would have opened the CB first before wanting to fiddle with a BBP CT on that feeder.

However in the principle of risk mitigation, our job is obviously not only to make sure that technicians are trained on how to follow procedures (and not make mistakes) such as verifying that the CT is not carrying primary current,
but also to make sure that our procedures anticipate them making mistakes!

People can get distracted or interrupted so it is easy to open one CB, even verify that there is no current in the CT primary, but then turn around and fiddle with the links of a different feeder.
i.e. if they open one CB to work on that feeder, but then erroneously start working the links on a different feeder, then the choice of Standard Operating Procedure (habits) of “short-then-open” or “open-then-short” is VERY IMPORTANT AND HAS VERY DIFFERENT HAZARDS.
In this mistake scenario of working on the wrong feeder, the outcomes are:

  • short-then-open may result in a BBP trip of the busbar
  • open-then-short may result in a BBP trip of the busbar but will also open circuit the CT, leading to potential flashover/explosion of the CT and potential injury or death to the technician

The scenarios are explained in this presentation. HiZ CT Isolations.pdf
You are free to use these slides as long as they are in their full form (my logo etc).

My general recommendation is to check what is your company procedures and consider if changes and re-education are necessary.

I appreciate that some may say "but it has been done successfully for years without incident", but this simply means on the statistics of "1-in-a-billion", the chances of it happening in the remainder of the "billion" are both non-zero and increasing.

If the worst came to happen, the next worst thing is to be standing in a Coroners Inquest and being asked to explain on one hand a procedure that errs on the side of safety for other than HiZ BBP CTs of short-then-open, but for HiZ BBP CCTs where Kneepoint voltages are often so much higher as well, that a procedure of open-then-short with more risks is mandated, perhaps even creating confusion for what may be the right procedure in other circumstances/conditions.


The final aspect to keep a close eye on is earthing of the secondary system.  It is important that each independent section of the secondary wiring is earthed at one point to prevent the circuits "floating" to high voltage levels with respect to earth. 

Depending on where that earth point is for the CT connections with respect to the isolating links and whether the opening of the links totally disconnects one side of the wiring from the other side, you may need to apply a temporary earth to the side that has no path to the permanent earth point. Clearly it is also essential to remove the temporary earth as the systems are being reconnected to each other.

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