Fuses, circuit breakers, and protective relays: switching, sensing, and interrupting devices

[akyip]

Hey guys,

So I was in a practice session, and a topic came up about whether fuses, circuit breakers, and protective devices are switching, sensing, and interrupting devices. I want to confirm that my understanding of these OCPD devices are correct.

Fuses are sensing and interrupting devices, but not switching deivces:

• Fuses have a thermal element that can sense increasing heat from high currents or over-currents.
  
• Fuses interrupt the circuit by melting and thus open-circuiting the circuit once melted.
  

Circuit breakers are switching, sensing, and interrupting devices:

• Circuit breakers can be manually switched on and off to make or break a circuit as needed.
  
• Circuit breakers have both a thermal element to sense increasing heat from overload, and an internal magnetic coil that activates and trips the circuit when there is a high fault current (because the high fault current produces a very strong magnetic field that triggers this magnetic coil).
  
• Circuit breakers automatically trip and interrupt the circuit when a current higher than the breaker's continuous current rating persists in the circuit.
  

Protective relays are only sensing devices:

• Protective relays use instrument transformers such as CTs and PTs to monitor current and voltage in a line or circuit.
  
• Protective relays send trip signals to OCPDs, but protective relays themselves are not interrupting or switching devices.
  

Is my understanding of all this correct? Feel free to correct me if I'm wrong.

 

[jd5191]

I'm no expert, but my two cents:

Fuses are certainly interrupting devices, I guess you could argue they're also sensing devices, but I normally think of sensing devices as meters, metering transformers or relays.

I wouldn't consider Circuit Breakers sensing devices. They may have thermal failure sensing, but in my experience the overcurrent protection devices are external to the breaker.

Agree on Protective Relays

 

[Dothracki PE]

Hey guys,

So I was in a practice session, and a topic came up about whether fuses, circuit breakers, and protective devices are switching, sensing, and interrupting devices. I want to confirm that my understanding of these OCPD devices are correct.

Fuses are sensing and interrupting devices, but not switching devices:

• Fuses have a thermal element that can sense increasing heat from high currents or over-currents.
  
• Fuses interrupt the circuit by melting and thus open-circuiting the circuit once melted.
  

Circuit breakers are switching, sensing, and interrupting devices:

• Circuit breakers can be manually switched on and off to make or break a circuit as needed.
  
• Circuit breakers have both a thermal element to sense increasing heat from overload, and an internal magnetic coil that activates and trips the circuit when there is a high fault current (because the high fault current produces a very strong magnetic field that triggers this magnetic coil).
  
• Circuit breakers automatically trip and interrupt the circuit when a current higher than the breaker's continuous current rating persists in the circuit.
  

Protective relays are only sensing devices:

• Protective relays use instrument transformers such as CTs and PTs to monitor current and voltage in a line or circuit.
  
• Protective relays send trip signals to OCPDs, but protective relays themselves are not interrupting or switching devices.
  

Is my understanding of all this correct? Feel free to correct me if I'm wrong.

This is a great list! Definitely would have put this in my reference binder (if I could have one). When you say circuit breakers, this seems to be in reference to thermal-magnetic breakers specifically, although I don't think protection gets into much detail with adjustable-trip electronic breakers except when dealing with coordination curves.

 

[akyip]

This is a great list! Definitely would have put this in my reference binder (if I could have one). When you say circuit breakers, this seems to be in reference to thermal-magetic breakers, although I don't think protection gets into much detail with adjustable-trip electronic breakers except when dealing with coordination curves.

Well, I want to make sure what I listed is correct lol.

It came up that apparently circuit breakers are not sensing devices and that protective relays count as switching devices...

I thought that circuit breakers would count as sensing devices, since they have the thermal element and magnetic coil... And I thought that protective relays themselves didn't really count as switching devices, since in my mind they only send out trip signals for OCPDs to trip.

Just want to make sure what I typed up is actually correct.

 

[Ampera18 PE]

I think not everything that has a time delay is a sensing device, which is why circuit breakers are not. 

I also would also like to add to the conversation that circuit breakers, while switching devices, really are not meant to be switches, and they have a certain numbers of switching in their lifetime.

 

[akyip]

I think not everything that has a time delay is a sensing device, which is why circuit breakers are not. 

I also would also like to add to the conversation that circuit breakers, while switching devices, really are not meant to be switches, and they have a certain numbers of switching in their lifetime.

I understand what you're saying about circuit breakers not really meant to be switches. I just find it odd that circuit breakers have thermal elements and magnetic coils that can be used to sense overcurrent, yet breakers themselves are not considered to be sensing devices. Especially since fuses can be considered as sensing devices.

I'm also not sure on the consensus on whether or not protective relays are actually considered as tripping devices.

 

[Ampera18 PE]

IDK is the melting of the fuse considered sensing? idk about that one

protective relays are tripping devices. they trip contacts and breakers. If you just have a Normal MV VFI, with no relay, it won't trip because it has no brains.

 

[Byk]

I understand what you're saying about circuit breakers not really meant to be switches. I just find it odd that circuit breakers have thermal elements and magnetic coils that can be used to sense overcurrent, yet breakers themselves are not considered to be sensing devices. Especially since fuses can be considered as sensing devices.

I'm also not sure on the consensus on whether or not protective relays are actually considered as tripping devices.

I don't think that fuses are consider to be sensing devices.

Regrading the protective relays, I don't think that they are consider tripping devices. The Relay does not break the contact it just senses the error and send the signal to the circuit breaker.

 

[akyip]

Bump.

If they're not too busy, I would like to hear some input from some renowned experts like @Zach Stone, P.E., @Cram For The PE, and @justin-hawaii on this: whether each protective device type (fuses, circuit breakers, and protective relays) count as switching, interrupting, and sensing devices...