coordination of breaker with the fused disconnects

[Wasanthimala]

As per Canadian electrical code(section38), the elevator needs a fused disconnect switch inside the elevator machine room. I have an application where, the elevator is fed from a circuit breaker in a remote panel and the fused disconnect switch(type D) is in the elevator machine room. The code guides me to calculate the size of the fuse and the clips. But How can I calculate the size of the breaker in the panel? My understanding is it can't be the same size as the fuse as it will give rise to a loss of coordination between the fuse and the breaker.
Thanks in advance for any guidance

 

[jghrist]

I can't speak to the code issues, but if the breaker feeds only the elevator, then coordination with the fuse is not an issue. If the breaker is not coordinated with the fuse, then it will trip before or at the same time as the fuse. No additional loads would lose service. In fact, it may be preferable for the breaker to trip before the fuse is damaged so that you avoid having to replace the fuse.

 

[kitha]

jghrist!
Then whats the meaning of the fused disconnect switch if the breaker does the same job? Why can't we use a non fusible disconnect instead? Is there anybody out there who could address the issue based on Canadian electrical code. I have seen in US the same situation(Breaker with a fuse) to the same feeder. Breaker is not only magnetic but thermal-magnetic.

 

[advidana]

If that is the only load on the circuit what difference does it make if it coordinated or not. Anyway, The fuse is down stream of the cb will blow before the breaker that is the way it should be. What is the problem, so is coordinated.

 

[jbartos]

Suggestion: The electronic trip circuit breakers may have the short circuit (or instantaneous) trip delayed depending on the electronic trip unit type. Therefore, the coordination between the suitable electronically tripped circuit breakers and a fuse downstream is possible.

 

[rbulsara]

kitha:

As jgrisht indicated, when loss of either device in series results in loss of the same load, coordination becomes immaterial. Opeing of the breaker before the fuse may be a inconvenience but it is NOT a safety hazard nor a code violation!

Your Code may require a fused disconnet but that does not preclude use of a breaker upstream of that.

No Codes defy engineering principles. They could, sometimes, be misinterpreted or over interpreted.
Sometemes Codes may become overzealous in inisting the safety than coveniences.

For example, the elevator inspectors may not want to look beyond the local disconnect to make sure that elevator drive and its wiring is adeuately protected and there is positive service disconnet in the vicinity to ensure saftety of person working on the elevator system for repairs. He would not care as to how the disconnect is fed or in case of a fault upstream device opens.

Speaking of physics or engineering, you would not be able to achieve coordinatoin between two devices of equal rating ( or closely rated). And yes, cricuit breakers in fact does the same job as the fuses (lets not get into their finer characteristics). Meaning, a 200A breaker protects a 200A rated condcutor and so does a fuse. But you also have the motor so they should be sized per the applicable seciton of codes for a motor branch cicuit.

Side Note: NEC (I beleive CEC is based on the NEC) suggests that for motor branch circuit protection, the following is applicable:

Non-time delay fuses: 300% max. (of FLA of the motor)
Time delay fuses: 175% max.
Thermal CB : 250% max.

 

[jbartos]

Suggestion: A selective coordination of protective devices is often specified in the project specifications. The circuit breaker is allowed to clear three short circuit faults before it loses the UL label. Then, it is not prudent to have a circuit breaker trip and a fuse blown because of one short circuit since after three short circuits the circuit breaker would have to be replaced.

 

[Ahr35181]

The reason for having a fused disconnect switch inside an elevator machine room is to allow for quick isolation if the lift breaks down in between floors, & therefore the rescuers can isolate at a local point with no need to lock off, as the point of isolation is visible whilst the rescuers manually operate the lift break & wind the lift up or down by hand.
I see no reason for having two points of fault / ovecurrent protection on one supply, from a maintenance point of view it would make life more dificult if there was no co-ordination between the cb & the fuses (i.e if they are the same rating). Maybe the Canadian standard should be simplified & require a disconect switch, then you could fit copper links in place of the fuses whilst having fault / ovecurrent protection at source.

 

[Pinemarten]

K.I.S.S.
Elevator...
Humans in elevator....
Local fusing for elevator...
If service personal, or rescue workers need to 'adjust' local fusing to make it move, then let them.

 

[dpc]

There are many instances where fused disconnects are required to protect "packaged" electrical equipment, regardless of the upstream protection. For elevators, there is an added safety element, where consistency in all installations may have benefits in an emergency situation. In the US, NFPA requires a shunt trip on the circuit breaker feeding elevators so they may be shutdown automatically by smoke detection equipment.

Many HVAC packages such as chillers, etc have long required fused disconnects for protection since this was the basis for their UL approval. The fuses can provide current-limiting that can help in short circuit tests. When it comes down a choice between doing something logical or following the code, you just follow the code, unless it appears to obviously unsafe.

 

[kitha]

My thank to dpc for broaching the issue on UL approval for Air-conditioning equipment.Would you please elaborate little bit on what basis the UL requires fused disconnect switches. I know in most of the european electrical codes there's no such a requirement amd the properly sized thermal magnetic circuit breaker installed at the distribution panel would do the job.However the power panel that comes with elevator, chillers etc has it's own disconnecting means(mostly a circuit breaker). According to wasanthimala's explanation, I have a doubt if there's any redundant disconnecting means installed by the electrical contractor thereby three protective devices for a single feeder.