DOUBLE FUSING

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timo

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I'M CONFUSED ON DOUBLE FUSING. I HAVE A 3 POLE ELEVATOR FEEDER DISCONNECT IN A DISTRIBUTION PANEL FUSED AT 90 AMPS. THEN IN THE ELEVATOR MACHINE ROOM THE LOCAL DISCONNECT IS A 3 POLE FUSED DISCONNECT, FUSED AT 90 AMPS. ISN'T THIS DOUBLE FUSING? WHAT SECTION OF THE NEC COVERS THIS? THANKS , TIMO
 
Re: DOUBLE FUSING

I forget what part of the code it is at, but if I'm not mistaken of the breaker/disconnect(at the main) is not within sight or more than 50' an equipment or what have you must have another form of disconnect.Example: Why do hvac have disconnect's by them and similar things? For that same purpose.

[ January 16, 2005, 02:51 PM: Message edited by: elect36 ]
 
Re: DOUBLE FUSING

Nope, selective coordination is not in Article 100. If I used my limited knowledge of Eglish I could guess that means I could opt to not coordinate it. :confused:

Edit: adjusment.

[ January 16, 2005, 04:17 PM: Message edited by: physis ]
 
Re: DOUBLE FUSING

Sam that term is used in regards to health care facilities also.

Here is the NEC definition

240.2 Definitions.
Coordination. The proper localization of a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault-protective devices.
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Basically it means a fault should only trip the overcurrent device nearest the fault.

Sometimes a fault on a 20 amp branch circuit will take out a feeder breaker along with the 20 amp breaker.

In the case of one feeder running a bank of elevators a fault on one elevator should not knock out the feeder main, only the affected elevator should go out of service.
 
Re: DOUBLE FUSING

Thanks Bob, I'm glad you added some additional explaination. The second half of the definition's as bad the choise of wording it's trying to define. :)
 
iwire said:
Sam that term is used in regards to health care facilities also.

Here is the NEC definition

Basically it means a fault should only trip the overcurrent device nearest the fault.

Sometimes a fault on a 20 amp branch circuit will take out a feeder breaker along with the 20 amp breaker.

In the case of one feeder running a bank of elevators a fault on one elevator should not knock out the feeder main, only the affected elevator should go out of service.
Click to expand...

Relating to the above thread, I have designed the electrical power distribution for a 3 story apartment building that contains 2 elevators of the hydraulic type at 208 volt 3 phase. Each is individually fed from a house service panel that contains a thermal magnetic main circuit breaker. Each elevator feeder circuit breaker in the house panel as well as its disconnect in the machine room is of the solid electronic trip circuit breaker type. Per NEC 620.62 I have completed a circuit breaker coordination study and find where I cannot coordinate the elevator feeder breakers with the house panel service main in the instantaneous region ( one cycle timeframe) at fault levels near the maximum available 3 phase symm fault currents. Any suggestions?
 
The machine room disconnect is required to be an OCPD. If it and the upstream device are the only two in series, I wouldn't worry about selective coordination. I believe that two 90 amp fuses in series would be OK.

Jim T
 
I have completed a circuit breaker coordination study and find where I cannot coordinate the elevator feeder breakers with the house panel service main in the instantaneous region ( one cycle timeframe) at fault levels near the maximum available 3 phase symm fault currents. Any suggestions?
Click to expand...
Use fuses for the main?
 
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bkaz said:
I have completed a circuit breaker coordination study and find where I cannot coordinate the elevator feeder breakers with the house panel service main in the instantaneous region ( one cycle timeframe) at fault levels near the maximum available 3 phase symm fault currents. Any suggestions?
Click to expand...

Make sure you are using the fault current available at the elevator breaker, and be sure it is accurate. It's easy to overestimate fault currents and have problems.

Jim T
 
pierre said:
We finished a large IBM building about 15 years ago, that had a rooftop 3hp - 480volt motor trip the 4000 amp main :D

Pierre
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Ground fault? I've seen that before where ground fault protection tripped before the relatively slow acting fuses or TM breaker on a motor branch.
 
Jim,
If it and the upstream device are the only two in series, I wouldn't worry about selective coordination. I believe that two 90 amp fuses in series would be OK.
Click to expand...
How can two devices of the same type and rating be selectively coordinated?
Don
 
don_resqcapt19 said:
Jim,

How can two devices of the same type and rating be selectively coordinated?
Don
Click to expand...

They can't be, but 620.62 only requires selective coordination if there are two or more devices downstream of the upstream device. If I have a breaker feeding the elevator equipment from a panel or switchboard, and then have the required OCPD in the elevator room, the two alone in series don't require coordination.

Opening that upstream breaker doesn't cause unnecessary outages.

Jim T
 
620.62 does not apply. 620.62 refers to ground faults, and that ground fault protection be coordinated. If there is a fault to ground outside in a landscape light, you won't take out the main breaker of the whole building.

Certain services are required to have Ground Fault Protection on the Main breaker. If so, there is distribution Ground Fault Protection that follows downstream. The coordination is required so that either the Branch Circuit (if present) or the Feeder Ground Fault Protection will trip in a Fault, instead of Higher up the distribuion stream, or even all the way up to the top.


Also, Fusing is not Ground Fault Protection. Fuses act as OverCurrent Protection and Short Circuit Protection. They do NOT sense Faults to ground. Yes, they will or may trip as a result of a fault, only because the resulting current due to the fault may trip the overCurrent Protection, but it is not Fault protection. 620.62 is in regards to FAULTS only.
 
As for the 'double' fusing. There is nothing limiting how many Fuses you put in a circuit. It is a bit redundant, if you are using it as a second set of fuses, but there may be circumstances that call for a second set of fuses in a circuit. ALso, if you have a fuse that blows, it means more places to search for a blown fuse, but nothing wrong with that either.



Try this one.
You have an FPE panel, and you want to install an A/C unit outside. Do you install a FPE HACR rated breaker? They may be out there, but I have never seen one. Instead, you can install a breaker to feed a fused disconnect. At the Fused disconnect, you can have your OCP and Short Circuit protection needed for HACR type equipment. This would be a fuse and a fuse in line, it is just that one of the fusing would be breaker instead of a fuse.
 
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milwaukeesteve said:
620.62 does not apply. 620.62 refers to ground faults, and that ground fault protection be coordinated. If there is a fault to ground outside in a landscape light, you won't take out the main breaker of the whole building.

Certain services are required to have Ground Fault Protection on the Main breaker. If so, there is distribution Ground Fault Protection that follows downstream. The coordination is required so that either the Branch Circuit (if present) or the Feeder Ground Fault Protection will trip in a Fault, instead of Higher up the distribuion stream, or even all the way up to the top.


Also, Fusing is not Ground Fault Protection. Fuses act as OverCurrent Protection and Short Circuit Protection. They do NOT sense Faults to ground. Yes, they will or may trip as a result of a fault, only because the resulting current due to the fault may trip the overCurrent Protection, but it is not Fault protection. 620.62 is in regards to FAULTS only.
Click to expand...

I don't see where 620.62 applies to only ground faults. Can you direct me to that information?

Jim T
 
Steve,
The word fault doesn't even appear in the code section.
620.62 Selective Coordination
Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.
Click to expand...
Don
 
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