To fuse or not to fuse?

[veritas]

My question primarily concerns LV MCC's but could certainly be applicable to HV MCC's as well. I have seen myriads of motor protection systems comprising of a cb in series with a fuse. Usual intention is that the fuse cater for the high current faults. I have noticed in many such applications (typically installed >10 years ago)that the cost of cb's with sufficient breaking capacity, to obviate the need for a fuse, cost quite a bit back then. However, nowadays they are getting substantially cheaper - relatively speaking and so I am wondering if fuse-cb arrangements are still being applied in new applications?

When I come across fuse-cb installations that require refurbishment I tend to go for cb only as it simplifies the installation provided that the cb has the correct fault current ratings. They also have better settings flexibility. I guess fuses have a lot going for them (such as no settings and being fast-acting) but there are also drawbacks. One installation had an incorrect fuse as when the original fuse operated, there was not an identical spare available and the circuit was never taken offline to fit the correct fuse since. Another had fuse contacts that pitted due to arcing and overheated resulting in a flashover.

I appreciate that there are pros and cons to both approached and am interested to hear other's experience/practice regarding this.

Thanks.

 

[quabillion]

I was told by an old timer that the fuse protects the motor and the breaker protects the wire.

 

[itsmoked]

Fuses are ready-and-able to single phase your motor loads.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Fuseless is the rule on this side of the pool. And has been so for long time. There is always "something"(cable, fuse or the impedance of the transformer) that limits prospective Isc to less than the breaker's interrupting ability.

quabillion: "I was told by an old timer that the fuse protects the motor and the breaker protects the wire"

In my reality, it is the other way round.


Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

I have always expected the motor overload to protect the motor.

Today, the series breaker and fuse would have to be tested to ensure the combination is properly co-ordinated.

 

[Skogsgurra]

Clarification: The breaker IS the thermal protection (settable trip for thermal protection) in some parts of the World. There is also an overcurrent trip, but that sometimes need fuses to limit current.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[veritas]

Current limiting fuses are of course able to limit the fault current to less than the prospective value by interrupting them within a quarter cycle. An LV CB typically needs at least 1 cycle to trip this rising to about 3 cycles at least for an HV breaker.

Aside - generally the relay/cb is meant to take care of overloads and lower range faults whilst the fuse sorts out the high short circuit currents. This does require the designer to ensure that the fuse does not expose the cb to the high fault currents - i.e. co-ordination required as was pointed out.

On the other hand, a modern LV cb with an electronic OCR allows greater flexibility for grading with upstream devices as well as tailoring the relay TCC to suit the motor starting characteristic and thermal damage curves. My vote goes for breaker only where possible if the circuit is able to withstand the If for the duration of the cb operating time.

Less hardware, cost and maintenance.