Contactor or fuse to interrupt short circuit? 3

[SELEC]

I am dealing with TOSHIBA international on a project. They have already purchased fuses for their switchgears. The problem is, their fuses can't ideally protect their contactors from short circuit faults. I understand that it is a common practce to allow fuses to blow rather to trip the contactors. But in our case, I am thinking about using SR469 to trip contactors first at the lower fault levels (within contactor interupting limits) solong as the contactors don't trip very often. Who has other options for this problems?

In addition, SR469 has a feature to block DC offset while starting a motor.

I appreciate it very much for any of your inputs.

 

[wbd]

If I understand you correctly, you want to protect the contactor contacts from a short circuit. This is what the fuses are doing. The contactor contacts are definitely not designed to interrupt fault current.
Since the National Electric Code requires overcurrent protection for contactors, most contactor manufacturer will provide recommendations on fuses to use with each size contactor.
These fuses should provide adequate protection for the contactors in the event of a short circuit.

 

[swgrmfg]

Mr. Selec:

You would be on the correct path if you were referring to the time inverse curve and not the instantaneous. If you set the time inverse curve of the 469 as you stated, then the R-rated fuses will perform their duty to clear the fault above the contactor interrupting rating. The trick is: do you have enough available fault current for the fuse to current limit (in a 1/4 cycle)? If yes, good. If not, rest assured, even if during an intermediate fault the contactor opens by itself because of voltage drop on its coil(s), the vacuum interrupters introduce a mere 200V drop in the short circuit path. Current keeps flowing (even with the interrupters fully apart)until the fuses clear. This has been time, true, tested for at least thirty years. Stay away from the Instantaneous - unless you use it as a blocking device!
Good Luck!

 

[rcw retired EE]

The 469 relay has a ground fault feature that can be implemented with a zero sequence CT encircling all of the phase conductors. It provides very sensitive GF protection. Since the motor is the last device in the circuit the GF protection is set to instantaneous or with a slight delay to prevent nuisance trips.

But, what happens when a phase-phase-ground fault occurs at the motor? Current could be above the contactor interrupting rating. The relay may try to open the contactor before the fuse has time to clear. Sometimes the GF must be delayed to allow fuse clearing. This creates problems with excessive coordination delays on upstream breakers.

This problem seems to only occur on large motors (4000 HP) or when we are being cheap and feeding a transformer with a latched contactor.

I have heard of a fatality when this happened and the contactor exploded, sending the front enclosure door flying. Welding of the contacts is a more common result.

So check the contactor integrity after any of the fuses blow.

 

[stevenal]

If I understand the question, you are trying to discriminate the low current faults from the high current ones, and wish to use the lower rated interrupting device to interrupt currents that happen to be within its rating. First of all, I see a possible code violation. NEC for example requires all overcurrent devices to be able to interrupt the available current. Second of all, faults evolve. Your contactor may be in the process of operating when the fault evolves to involve more phases for example. Past the point of no return, it is now trying to interrupt the higher current fault.

 

[SELEC]

Mr Swgmfg,
Soory for the late response.In 1/4 cycle, we have a large current to blow the fuse. The contactor can withstand peak current up to 85kA. I think the fuse sould be able to trip the circuit within the peak period so that the contactor wouldn't withstand longer high current.

 

[cuky2000]

Here is a generic motor, starter, fuse & relay coordination curves that may be used as generic guide.

I hope this could help.

 

[SELEC]

Mr Cuky2000,

That's what I intended to achieve at the beginning.But the lead engineer didn't agree and he insisted on tripping the fuse first.
You mentioned the fuse opens before contactor. But to me, motor relay will trip the contactor first under the fault current unless the relay is set before the minimum fault level.

 

[rbulsara]

May be this will help..
In a motor starter:

Overload relays are just that to protect agianst overload not fault current.

Fuses and circuit breakers are short circuit protection devices and they are intended only to interrupt heavy fault current and not overload.

Relay/contactor combination is there to make or break under normal load conditions and overload condiitons and not to interrupt fault currents.

Overload realys by design are slow reacting devices to overloads and too slow for fault current. While fuse/breakers will not react too quickly to low level overloads but they will act much faster than overloads relay if the current is very excessive as it would be in short circuit condition.

So for low level overload overload relay/contactor will (or should) open fisrt and in case of a short circuit the fuse or breaker will (or should) open first.