Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day concerns loop impedances on motor circuits and MCB operation. It is answered by SELECT:
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Question: I'm measuring loop impedances on motor circuits, which are three-phase - the Zs reads 10 ohms, protected by a 6A D-type MCB at the isolator. R1 + R2 reads 0.09 ohms and is OK - for the type of MCCB, the Zs reading does not comply.
After several tests, it was found that the outgoing cables from the control panel through the starter overloads were giving these results. The resistance of the /o/ls measures 10 ohms, the isolator is remote from the plant room main control panel, and the Zs remains 10 ohms. Will this still operate the MCB in the required time permitted?
Answer: In this instance, overload protection for the equipment downstream of the starter is provided by the starter overloads - and you would be expecting these to operate to protect the equipment.
I presume that you have confirmed the adequacy of the earth - i.e., the Zs at the isolator is satisfactory?
The circuit breaker in this instance is, I think, providing fault protection only for the part of the circuit from the isolator to the motor, and will therefore require you to ensure that it provides fault protection for the level of fault current determined at that point (i.e., the isolator).
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