To fuse or not to fuse?
To fuse or not to fuse?
(OP)
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.
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.






RE: To fuse or not to fuse?
RE: To fuse or not to fuse?
Keith Cress
kcress - http://www.flaminsystems.com
RE: To fuse or not to fuse?
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
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: To fuse or not to fuse?
Today, the series breaker and fuse would have to be tested to ensure the combination is properly co-ordinated.
RE: To fuse or not to fuse?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: To fuse or not to fuse?
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.