Suitability of using fuse and circuit breaker in the same circuit

[Deborah90]

Hello, I am working on a project that entails powering a bunch of 240VAC heaters through a power controller. The power controller requires a fuse that meets 200kA SCCR. However, since the heaters will be powered by 240 (VL-L), I am concerned that in case of a short circuit fault, both fuses on each line will not blow simultaneously. To mitigate the issue of meeting the SCCR requirement and ensuring both line trip at the same time, I am considering placing a circuit breaker downstream of the fuse. Hence, the fuse will serve to protect the power controller. Kindly advise on the suitability of my purposed solutions. Thank you.

 

[dpc]

It it not a Code issue, at least in the US. But it is possible, at extremely high values of fault current, that the fuses could operate so quickly, that the breaker will not trip. What is the actual available fault current?

 

[che12345]

Deborah90 (Electrical)(OP)19 Oct 23 16:59
"...#1. a project that entails powering a bunch of 240VAC heaters through a power controller. The power controller requires a fuse that meets 200kA SCCR. #2 ...the heaters will be powered by 240 (VL-L), I am concerned that in case of a short circuit fault, both fuses on each line will not blow simultaneously. #3....To mitigate the issue of meeting the SCCR requirement and ensuring both line trip at the same time, I am considering placing a circuit breaker downstream of the fuse. Hence, the fuse will serve to protect the power controller".
I have the following opinion for your consideration:
1. It is very unlikely that the actual short-circuit current level at the power controller location is as high as 200kA. Consult the controller for the recommended fuse to be used. The fuse that meets 200kA SCCR may apply to protect the electronic devices. In that case, a semi-conductor fuse would be needed. It is not replaceable by a normal general purpose fuse.
2. The source is L-L. A L-L short-circuit should blow both (i.e. two) fuses. However, it is possible that one of them blows to open-circuit with the other over-heated/injured, but not fully broken (open-circuit). Attention: Replace BOTH fuses even one of them may measure continuity.
3. If you are not using the semi-conductor fuse for protection of the electronic devices, you my use a double pole 160kA or 200kA current limiting breaker alone, without having the fuse ahead at all.
3.1 Consider the use of current limiting breaker with very low i² t, which is very close to special semi-conductor fuse characteristic.
Che Kuan Yau (Singapore)

 

[9wearSuave]

If the available fault current is higher than the planned breaker AIC you’d need to make sure the fuse and breaker combo can be series rated.

 

[RRaghunath]

Using a contactor downstream of fuses which is powered from the same supply should help.
The contactor will drop off isolating the faulty heater even if only one fuse has blown.
Use of an adequately short circuit rated (for the location) MCCB , with no fuses/contactor sould also work.

R Raghunath