I’m looking at a 13.2 kV primary switchgear with with three radial feeds, each with 3 #1/0 AWG aluminum conductors. These are rated for 155A. Each radial feed has Cutler Hammer RBA 400 100E fuses. The feeds power a series of distribution xfmrs on a campus. I came to realize that E fuses melt at 2 to 2.4X their rating after 300 seconds which has left me perplexed. Is this feeder protected? Is there a code or standard that permits a fuse to exceed the rating of the feeder? I’m worried about one of the feeds drawing 175 Amps with no forseable way to clear an overload. Any input would be appreciated.
Primary Switchgear Fuse Rating
- Thread starter strap89
- Start date
- Status
these fuses are designed for over-current protection (short circuit)
not timed over current (thermal or 'overload')
http://www.sandc.com/globalassets/s...ocuments---all-documents/tcc-number-153-4.pdf
APPLICATION—Like all high-voltage fuses, these refill units are intended to accommodate overloads, not to interrupt them. Accord- ingly, they feature fusible elements which are designed with a min- imum melting current of 200% of the refill-unit ampere rating (for refill units rated 100 amperes or less) or 220% of the refill-unit ampere rating (for refill units rated over 100 amperes). As a result, these refill units have considerable peak-load capabilities; however, they should never be exposed to loading in excess of the peak-load capabilities listed in S&C Data Bulletin 240-190.
not timed over current (thermal or 'overload')
http://www.sandc.com/globalassets/s...ocuments---all-documents/tcc-number-153-4.pdf
APPLICATION—Like all high-voltage fuses, these refill units are intended to accommodate overloads, not to interrupt them. Accord- ingly, they feature fusible elements which are designed with a min- imum melting current of 200% of the refill-unit ampere rating (for refill units rated 100 amperes or less) or 220% of the refill-unit ampere rating (for refill units rated over 100 amperes). As a result, these refill units have considerable peak-load capabilities; however, they should never be exposed to loading in excess of the peak-load capabilities listed in S&C Data Bulletin 240-190.
look at it like this
suppose you have a 100 Ohm ph-ph short
132 A at 100 ohm
1.7 MW
in a very small area
it won't be a high Z fault for long
also the cables likely have shields to ensure a ground fault if damaged
your 'overload' protection should be on the xfmr sec or possibly the prim
but proper load calcs and sizing are your real protection
suppose you have a 100 Ohm ph-ph short
132 A at 100 ohm
1.7 MW
in a very small area
it won't be a high Z fault for long
also the cables likely have shields to ensure a ground fault if damaged
your 'overload' protection should be on the xfmr sec or possibly the prim
but proper load calcs and sizing are your real protection
Interesting, so you you can treat this almost like a motor and only protect the feeder from a fault and not an overload? Does the NEC make comment on this?
Please see Art. 450.3.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
This situation will come up much more frequently in NESC (POCO) territory rather than in NEC territory.
I see, so you size the primary for overcurrent protection and to not trip upon inrush. Is there any issue with a 100E fuse protecting a 3-#1/0 AWG conductors?
I am referring to my log-log graph of both 1/0 AWG and a 100E fuse and see no problems.
1/0 AWG is rated about 150 A and the 100E fuse has a total clearing time of about 600 seconds at a current of 195A (195/150 = 130%). It's your call if allowing a 30% overload for 10 minutes would meet your criteria.
smoothops10
Member
- Location
- FL
- Occupation
- EE
240.101 requires the fuse shall not exceed three times the ampacity of the conductors.
Is the distribution system compliant with 215.2(B)(1)? Calculate the transformer primary current from the nameplate kVA of each transformer on each feeder. If the sum of the currents is less than the ampacity of the conductors it is. Following this rule essentially provides your overload protection. 215.2(B)(1) is often not complied with in my experience and IMO it is due to ignorance of the requirement.
Is the distribution system compliant with 215.2(B)(1)? Calculate the transformer primary current from the nameplate kVA of each transformer on each feeder. If the sum of the currents is less than the ampacity of the conductors it is. Following this rule essentially provides your overload protection. 215.2(B)(1) is often not complied with in my experience and IMO it is due to ignorance of the requirement.
- Status