Alex Mackie asks whether a painful electric shock received from a transformer on a building site was the result of a build-up of static electricity. His question is answered by SELECT (Theme – Fire & Safety):
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| Static shock from building site transformer? |
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Question: Recently we had a problem on a construction site when a worker complained that he had received a shock from the casing of a 10kva 440V - 110V transformer. He maintained that when he went to move the transformer, a spark went from the transformer to his hand and left his hand very sore.
The transformer in question is connected via a 30mA RCD and backed up with a 100mA RCD. On testing, the 30mA RCD did not trip within its parameters, but the 100mA unit did.
The transformer was visually and electrically tested and no fault was found.
a) Surely, if the casing had been 'live' the RCD would have tripped?
b) If the casing had been live, would you see a spark?
c) Would this not actually have been static electricity?
Answer: It is difficult to say without knowing all the circumstances and carrying out testing.
You state that the transformer is connected via a 30mA RCD backed up by a 100mA RCD. This suggests that the RCDs are on the primary side of the transformer, which will be ineffective for any secondary side circuit fault protection.
Did you fully test the secondary circuits?
It is possible that the worker had picked up some static, which could create a spark when the static was discharged to earth, but I would consider this unlikely to have left pain in the hand.
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