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Earthing and short circuiting HV bushings of transformers
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Earthing and short circuiting HV bushings of transformers
Hi all,
We are struggling to find rated equipment to attach to the HV terminals of a transformer to meet the requirements of the SM-EI.
SM-EI EE 5.1121
(3.732)
Before working on the windings or terminals of a distribution transformer (with secondaryvoltage less than 1000V), the transformer HV shall be isolated, and the HV terminals shall be earthedand short-circuited.
If the cable connected to the HV bushings is earthed and short circuited then electrically the terminals will also be earthed and short circuited, is this enough to satisfy this rule?
We are also using “Spider earths” at the terminals which is a spring loaded clamp to bond the cable for disconnection (removes any induced voltage) but these are not rated under fault conditions and don’t meet the “earth” requirement of SM-EI.
How are others managing this?
Regards,
Mark
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4 Replies
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Hi Mark.
Totally agree with the earthing and short circuiting at the HV end of the cable. If its earthed then the bushings it is connected to are earthed. We use Catu Earths at the bushings to short circuit.
Cheers, Rodie.
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Thanks Rodie
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Most of our cable connected transformers are fed via Magnafix switch units (MSU). We remove the fuses from the transformer fuse unit at the MSU and earth up the cable to the transformer at the MSU with earths specifically designed for this purpose. Other transformers fed by switches or CB’s can be earthed with the integral earth on the switchgear or hand earths specifically designed for the switchgear.
Pole mounted transformers are most often earthed up as explained by Rodie.
The LV side of the transformer must also be isolated to remove any risk of a backfeed.
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Great thanks for that Scott
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