Recently, I was asked to rewire a time clock ( 120v clock motor controlling two 120v loads )in parallel with a spring wound timer used as override and replace two of the three Class T fuses used as main protection for the panel ( 200a, 120/208v ). The person who asked me to do this wanted to know why the fuses opened and not the branch breakers for the two loads. MC cable was used as the wiring method so I'm assuming a short between grounded and ungrounded conductors. This being the case. Is it typical for a main fuse to go and not the branch breaker?
Overcurrent Protection
- Thread starter tilk
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charlie
Senior Member
- Location
- Indianapolis
Re: Overcurrent Protection
It really depends on the time/current characteristics of both devices and the amount of fault current available. As you have seen, it is possible.
It really depends on the time/current characteristics of both devices and the amount of fault current available. As you have seen, it is possible.
wirenut1980
Senior Member
- Location
- Plainfield, IN
Re: Overcurrent Protection
It looks like the fuses for the main panel were not properly coordinated with the downstream breakers. You need to try and get your hands on the time-current curves from the manufacturer for fuses and breakers and compare them to see what event (i.e. what magnitude of overcurrent) could cause the fuses to blow before the breakers could open.
It looks like the fuses for the main panel were not properly coordinated with the downstream breakers. You need to try and get your hands on the time-current curves from the manufacturer for fuses and breakers and compare them to see what event (i.e. what magnitude of overcurrent) could cause the fuses to blow before the breakers could open.
Re: Overcurrent Protection
I strongly suspect that there was a bolted fault which is not something that is easily coordinated.
For example should you have 10ka of available fault current and there is a bolted fault, all devices in series with that fault will see an instantaneous current of 10ka (not considering the impedance of the circuit which could reduce that a bit.) Because the current is instantaneous all OCPDs will see that same peek current. If you compare the trip curves of the devices you will find that there is no way that they will coordinate and it will become a race as to which one will clear the fault. It is not uncommon for the upstream device to tip which we would not like to happen which often seems unreasonable to us.
I strongly suspect that there was a bolted fault which is not something that is easily coordinated.
For example should you have 10ka of available fault current and there is a bolted fault, all devices in series with that fault will see an instantaneous current of 10ka (not considering the impedance of the circuit which could reduce that a bit.) Because the current is instantaneous all OCPDs will see that same peek current. If you compare the trip curves of the devices you will find that there is no way that they will coordinate and it will become a race as to which one will clear the fault. It is not uncommon for the upstream device to tip which we would not like to happen which often seems unreasonable to us.
Ed MacLaren
Senior Member
Re: Overcurrent Protection
If the downstream OCPD was a fast-acting fuse instead of a circuit breaker, selective co-ordination would be much easier to achieve.
Circuit breakers are inherently slower than fast-acting fuses.
Ed
If the downstream OCPD was a fast-acting fuse instead of a circuit breaker, selective co-ordination would be much easier to achieve.
Circuit breakers are inherently slower than fast-acting fuses.
Ed
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