MV cable protection

[mull982]

I am curious how the maximum values allowed for MV breakers and fuses are establisted for protecting MV cables. I know the NEC allows a breakers long time pickup to be up to 6x the MV cable ampacity and fuses are allowed to be applied up to 3x the MC cable ampacity.

Why for MV cables are these overload tolerances much greater than those of LV cables?

Why are breaker long time pickups allowed up to 6x cable ampacity while fuses are only allowed at 3x ampacity?

Are using the 3x and 6x guidelines good practice for protection of MV cables or are there rules of thumbs that typically dictate lower protection of MV cables?

 

[kingpb]

First, I believe your reference is to 240.101(A) which is for additional requirements for feeders. This would not apply to branch circuits as prescribed by 240.100.

Second, MV equipment is rated 100%, as the devices that act as protection are either relays or fuses and therefore when sized properly, and coordinated using time-current analysis, I have not seen where the Long-time (overcurrent) setting, by following the requirements of 240.100 would necessitate a setting greater than the cable ampacity.

It would be interesting to hear anyone from someone that has an example of where that might apply.

 

[jimmac49]

I am curious how the maximum values allowed for MV breakers and fuses are establisted for protecting MV cables. I know the NEC allows a breakers long time pickup to be up to 6x the MV cable ampacity and fuses are allowed to be applied up to 3x the MC cable ampacity.

Why for MV cables are these overload tolerances much greater than those of LV cables?

Why are breaker long time pickups allowed up to 6x cable ampacity while fuses are only allowed at 3x ampacity?

Are using the 3x and 6x guidelines good practice for protection of MV cables or are there rules of thumbs that typically dictate lower protection of MV cables?

Hi if you look at the notes in Table 310.64 (2005 NEC) they refer to percentage of insulation for some MV cables. We used alot of 35Kv. class EPR cable with 133% insulation as out going feeders to Unit Substations all over the facility yards. These cables had several temperature ratings..
1) Maximum operating temp. -90c
2)Maximum overload temperature.-130c (Not to exceed one hundred hours / year). Over load periods shall not exceed 5.
3)Maximum short circuit. 250c.

All out going 1200 amp. vacuum circuit breakers that fed these circuits had 50/51 relay protection.
Typically there is a coordination and short circuit study done by an Engineer that would take into account an entire overview of the entire system. They would the provide the relay settings and a host of other information and settings.
MV cable like eveything else has an expected life. The more heat you have the more cable degredation there is, so you wind up with a premature cable failure.

 

[jim dungar]

Why are breaker long time pickups allowed up to 6x cable ampacity while fuses are only allowed at 3x ampacity?

MV Breaker, actually relays, and fuse curves are created using slightly different criteria. The result is there is a slight difference to how the devices will 'mimic' cable heating. hence the difference in NEC allowable sizes. For a simple example: MV fuses curves are based on the fuse being at less than operating temperature. Another example is: different style breakers take different amounts of time to 'clear', you may have heard of 3-cycle, 5-cycle, or 8-cycle devices.

 

[erickench]

You say the NEC allows 6X long time for cable ampacity? Where do you see that? 6X sounds like motor starting current. The short time pickup would be 6X and not the long time. Long time is usually used for protecting cables at their ampacity or overload.