B vs C Curve Breakers with an SSR

[fifty60]

After looking at the trip characteristic curves of B,C, and D type breakers, I think that B curve would be the best breaker to use for Heater applications. The time delay for these breakers is the minimum of the 3, and for a purely resistive load this is desirable.

The only problem I can see is when the purely resistive Heater load is switched on and off quickly by an SSR. Would such an application where the heater load is being controlled by a fast switching SSR actually need to be protected by a C-Curve breaker?

Does the Solid State Relay control even make a difference to the characteristic curve breaker I should choose? I know if the OCPD were a fuse instead of a breaker, I would not even consider any kind of time delay.

 

[Jraef]

After looking at the trip characteristic curves of B,C, and D type breakers, I think that B curve would be the best breaker to use for Heater applications. The time delay for these breakers is the minimum of the 3, and for a purely resistive load this is desirable.

The only problem I can see is wn the purely resistive Heater load is switched on and off quickly by an SSR. Would such an application where the heater load is being controlled by a fast switching SSR actually need to be protected by a C-Curve breaker?

Does the Solid State Relay control even make a difference to the characteristic curve breaker I should choose? I know if the OCPD were a fuse instead of a breaker, I would not even consider any kind of time delay.

No, it makes no practical difference using typical heating elements such as no-chrome, Use the B curve. The only issue is if you know that the heating element has a very high PTC ratio of resistance, meaning the resistance is extremely low until it heats up. Quartz lamp heaters are sometimes like that, so there is an inrush of current when you first turn them on if they are cold. Usually that can be controlled by ramping the SSR but if you have a cheap bang-bang SSR that cannot ramp, AND a high PTC ratio element, a D curve may be in order.

 

[fifty60]

I agree. Regarding sizing Contactors and SSR when using these breakers. Should I size the SSR's to the instantaneous trip value of the Breaker? I have heard that standard OCPD will not protect a normal SSR, and that a semiconductor type fuse should be use. Using this info, I would think that upsizing the SSR to the instantaneous trip value of the breaker would not help. Plus, this approach would cause very large SSR's to be used every time. When using D-curve breakers, should I take extra caution with sizing Contactors, or will the contactors still be protected by the thermal overloads of the motors? To sum up my question, what precautions do I need to take sizing everything else in the branch circuit when I choose a particular curve characteristic(B,C,or D?)

 

[templdl]

Before trying to apply a UL1077 device make sure that it complies with NEC art 241.10. UL1077 devices are not to be used as a substitute for branch circuit over current devices.
Quite often a component is address but the system that it is placed in is overlooked. I would understand that there is a UL489 upstream from the UL1077 supplementary protector.

 

[Jraef]

I agree. Regarding sizing Contactors and SSR when using these breakers. Should I size the SSR's to the instantaneous trip value of the Breaker? I have heard that standard OCPD will not protect a normal SSR, and that a semiconductor type fuse should be use. Using this info, I would think that upsizing the SSR to the instantaneous trip value of the breaker would not help. Plus, this approach would cause very large SSR's to be used every time. When using D-curve breakers, should I take extra caution with sizing Contactors, or will the contactors still be protected by the thermal overloads of the motors? To sum up my question, what precautions do I need to take sizing everything else in the branch circuit when I choose a particular curve characteristic(B,C,or D?)

You size the control device for the load, not the breaker. No breaker is going to act fast enough to protect the solid state components, in fact I am of the opinion that "semiconductor" fuses are a waste as well. I have used them for 30+ years on solid state controllers, yet not ONCE have I seen a fuse clear fast enough to prevent damage to a device that is misfiring. The fuse blows, but AFTER the SS device shorts. So now you have to replace the SS device AND an expensive fuse. I'm not saying don't use fuses, I'm saying that spending extra for specialized fuses that claim they will save the semiconductor devices is a waste.