Neutral Point of Heater Bank

Assuming the heating elements are resistive loads, there are three basic single-fault failure modes:


(a) Open-circuit element. The voltage drops on the other two elements to 200 V each.


(b) short-circuit to Earth. THis one is more tricky, as it may well not be a "fault of negligible impedance" as assumed by BS 7671 ... and overcurrent protective device may not operate. It depends how the break happens, but either the protective conductor connection restores Neutral through the earth... and the overcurrent protection might not operate. The shortened element could then eventually blows open-circuit, and rapiring the lement will then remove the "star-point to Earth" fault.


(c) shortened element. (resistive element resistance decreases). This will put an overvoltage on the other two elements, and they are potentially at risk of over-voltage and damage.



So I guess, in most of the failure modes, resistive elements wouldn't require a Neutral, but just a caution that element faults don't always follow the assumptions made in BS 7671.

Assuming the heating elements are resistive loads, there are three basic single-fault failure modes:


(a) Open-circuit element. The voltage drops on the other two elements to 200 V each.


(b) short-circuit to Earth. THis one is more tricky, as it may well not be a "fault of negligible impedance" as assumed by BS 7671 ... and overcurrent protective device may not operate. It depends how the break happens, but either the protective conductor connection restores Neutral through the earth... and the overcurrent protection might not operate. The shortened element could then eventually blows open-circuit, and rapiring the lement will then remove the "star-point to Earth" fault.


(c) shortened element. (resistive element resistance decreases). This will put an overvoltage on the other two elements, and they are potentially at risk of over-voltage and damage.



So I guess, in most of the failure modes, resistive elements wouldn't require a Neutral, but just a caution that element faults don't always follow the assumptions made in BS 7671.