Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, on lighting systems and varying answers to an exam question, is answered by Philips Lighting:
Question: I am currently re-training to become a sparkie. I become concerned when I ask the same question to different lecturers at college and receive different answers every time. I had to answer the following question in an exam and queried the answer later, as I was unsure about it. What do you say?
An industrial 11kW lighting system wired in four-core cable is installed. If one phase was to fail, what would be the result?
1) Nothing would happen - the lights would keep working.
2) The current in the neutral would increase.
3) All the light would go out.
4) The current in the neutral would decrease.
I thought answer four would be correct, because if they were operating on three separate phases, then demand would fall on the failed phase. One person agreed and another said it was, in fact, answer two, because the system would become unbalanced.
If answer two is correct, could you please explain why this is so when the demand has fallen? Surely this would mean that if a house had its supply at the mains turned off, then that whole system would be more unbalanced. Is this the case?
Answer: This is more of an electrical question than a lighting question, and you may wish to clarify it further with the IEE.
However, in answer to your question, given three-phase operation on the basis that this is a four-core cable, if one phase fails then the neutral current resulting from the other two phases still remains below the current rating of the neutral cable. The current will increase, but it will remain below the current rating of the cable. The lights would remain working on the two remaining phases.
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