Insulation testing of LV switchboard

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CParry

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Hello,
When I do a 1000V insulation test on a LV switchboard, shall I disconnect the CTs or measuring devices from the test circuit?
 
The instruction manuals for each device will list the terminals dielectric strength. If you cannot find them, then it is always safest to disconnect. This can lead to wiring errors when reconnecting.
 
If this is 600V rated equipment I would absolutely leave them in place. That stuff is supposed to withstand a continuous peak voltage of ~850V. If a 1000V megger causes it to fail it was unsuitable for service.
 
The issue is the CTs can't be removed without disassembling the switchboard.
Will it be safe to do the DC megger test on the busbar while the secondary of the CT of the busbar is open circuited?
 
CParry said:
The issue is the CTs can't be removed without disassembling the switchboard.
Will it be safe to do the DC megger test on the busbar while the secondary of the CT of the busbar is open circuited?
Click to expand...

CT's should never be left with the secondaries open-circuit. If the load wires are disconnected then place a jumper across the sec terminals. As Big John said, whether in-line or doughnut type they are designed for the 1KVDC test voltage.
 
Since CTs do not make any conductive connection to the wires, if the wire insulation is good for 1kV the CT construction should not matter.
But better safe than sorry.

mobile
 
CT Concerns

CT Concerns

CParry said:
Hello,
When I do a 1000V insulation test on a LV switchboard, shall I disconnect the CTs or measuring devices from the test circuit?
Click to expand...

Meggering the bus with the CTs in place has two problems. The first was already discussed in that you should ensure the CT secondary is short circuited. Second, 1000 Volt DC testing may leave the CT's polarized and may cause an operation immediately upon energizing. To avoid this, I suggest AC high potential testing where CTs are left in the circuit.

Hope this helps,

Newton Law
 
Having CTs open circuited is only a concern when there is alternating current flowing through the primary. It would not matter during an insulation resistance test, just as long as the secondaries were reconnected prior to turning the gear back on.
 
NewtonLaw said:
Meggering the bus with the CTs in place has two problems. The first was already discussed in that you should ensure the CT secondary is short circuited. Second, 1000 Volt DC testing may leave the CT's polarized and may cause an operation immediately upon energizing. To avoid this, I suggest AC high potential testing where CTs are left in the circuit.

Hope this helps,

Newton Law
Click to expand...

I do not see how DC voltage (and no current) applied to the conductor could "polarize" the CT wires or core to the point that they could cause a differential current on the secondary when the conductor is re-energized.
 
big john said:
Having CTs open circuited is only a concern when there is alternating current flowing through the primary. It would not matter during an insulation resistance test, just as long as the secondaries were reconnected prior to turning the gear back on.
Click to expand...
Yes, you are correct. But it has been our experience that the person starting the job may not be the one that finishes it and to avoid burnt up CTs the safest practice is to keep them shorted till the load wires are connected.
Another reason is if the gear is energized with the secondaries open-circuited, it presents a safety hazard: extremely high voltage at the CT terminals. This is why with new equipment they are always shorted at the factory before the gear is shipped out.
 
GoldDigger said:
I do not see how DC voltage (and no current) applied to the conductor could "polarize" the CT wires or core to the point that they could cause a differential current on the secondary when the conductor is re-energized.
Click to expand...
Yeah, I don't see it either. The CTs are coupled to the power circuit via the expanding / contracting AC fields created by CURRENT flow in the circuit, they don't know or care what the voltage is, and even if there were some current flowing in the DC circuit, it would only expand ONCE. Your hi-pot tester likely has fuse protection at something like 2A, so not even that much current would flow and would barely even register on a CT.
 
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