fused cutout as a service disconnect 2

[jcroley]

I have a question on what qualifies as a service disconnect. I have a 25kv service feeding a 15000 KVA transformer. The utility company is providing the primary underground, but the transformer (customer owned) has top bushings and I have to transition up a pole and feed it from overhead. The NEC says that a disconnect is required and that no more that 6 six throws of the hand to turn off all power. Would three (one per phase) pole mount fused cutouts qualify as this means of disconnect? This seems to be the norm for utilities. Any advice is appreciated.

Thanks,

Jim

 

[DistributionGuru]

Are you sure the TX is 15000kVA? That's rather large for fuse protection. Normal fuse protection is up to 1500kVA Tx.

 

[jcroley]

Yes, it is a 15000 kva xfmr. If not fuses, what do you recommend? This is a solution that S&C recommended.

Thanks,

Jim

 

[dpc]

I don't see how this could satisfy NEC service disconnect requirement. Service disconnect must be readily accessible. And it has to be a load-break device.

You will need a metal-enclosed fused switch or maybe a pad-mounted switch operable from the ground (without a hotstick).

For a 15 MVA transformer, you really need a primary circuit breaker or circuit switcher.

 

[davidbeach]

Fuses are certainly possible at 15MVA, but why? Obviously S&C would recommend fuses for this application since they are a fuse manufacturer. I wouldn't use fuses though. I'd put in a circuit switcher with a air break disconnect switch.

Fuses mean that all you have is phase overcurrent protection, and high side fuses do not do a good job of protecting against low side faults, particularly against low side ground faults behind a delta-wye transformer. A circuit switcher would also mean that you have something to trip for faults between the transformer and the secondary gear. This will help tremendously with arc flash mitigation.

 

[SteveWag]

I doubt that the transformer disconnect would fall under NEC, but if it does, the NEC requires that it be labeled “Suitable for use as Service Disconnect”.
Steve

 

[rbulsara]

Six disconnect rule at 25kV, 15MVA transformer?

Start with your local inspector see what he/she says and post it here. You can also send an email to NFPA (you need to become a member) and ask for informal interpretation.


Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

NEC 230.205: "Each service disconnect shall simultaneously disconnect all ungrounded service conductors that it controls and shall have fault closing rating that is not less than the maximum short circuit current available at its supply terminals."

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[BJC]

The logic behind the rule is that a fireman can find and shut down power to a building. Few ( none I hope ) inspectors would buy into a fused cutout as a disconnect. A secondary a circuit breaker with a remote switch is the way I would go. Depending on locaion of the transformer I would use a high sied circuit switcher.
When they show up to put your fire out they want a dead building. If they have to aim the water at just the secndary side of the transformer they will just watch.

 

[alehman]

An anecdote to go with BJC's response: There was a fire reported in a telecom switching center. The fire dept arrived and asked if anyone was in the building. They were told no, and thus responded that they would not be going in until the smoke cleared. And those places have pre-arranged power procedures sequences for just such situations.

You need a readily accessible and properly rated service disconnect that doesn't require qualified personnel to operate.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[steelerfan28655]

Where is the metering point? Typically the utility will provide fault protection up to that point.
Does the transformer have internal fusing, or protection?
Absolutely check with your local inspector to see if he will even take on the responsibility of looking at it.

As a utility, we have customers that are primary metered, but we still provide the protection and disconnecting means for each transformer. Although the largest transformer on our system is a 1500 kVA at 12,470. We use fused cut outs for our disconnects. Few companies and fire departments have personnel qualified to us a hot stick to open fused cutouts, particularly at 25 kV. They are manually operated, so they cost less than a relay controlled circuit switcher or breaker. Our 15kV, 600 amp switches cost us around $2,000.00.

Try looking at gang operated switches that are load-break and load-make rated. They do not provide fault protection, but a single operation will open all three phases simultaniously so you don't have to deal with single-phasing on your large motor loads.

Happiness is a way of travel, not a destination.

 

[steelerfan28655]

THis is why you should proof-read your posts before submitting them.
"They are manually operated, so they cost less than a relay controlled circuit switcher or breaker. Our 15kV, 600 amp switches cost us around $2,000.00."
This refers to the gang operated switches, not the fused cutouts.

It should have read like this:

Try looking at gang operated switches that are load-break and load-make rated. They do not provide fault protection, but a single operation will open all three phases simultaniously so you don't have to deal with single-phasing on your large motor loads.They are manually operated, so they cost less than a relay controlled circuit switcher or breaker. Our 15kV, 600 amp switches cost us around $2,000.00.


Happiness is a way of travel, not a destination.