Fusing for Stepdown transfomers

On a job site , where we have 2 460/480 stepdown transformers. Once supplies 120 VAC for controls(1,5KVA), other supplies 230 VAC (3KVA)for an MCC panel air conditioner. Plant 480 is actually sitting @ 500VAC, but customer has noted other 460/480 feed controls have been operating for years with not issues.

Tonight, the fusing to both of the step down transformers blew on MCC panel powerup. Fusing amps has been properly calculated and all week had no trouble until today.

Only way I could get my 460 / 120 VAC transformer to stop blowing fuses was to up rating from 3a to 6a. Unfortunatley I have no larger fuses than 6a . The 3KVA transformer had 7.5a fuses, and even with no load, the 6a blow. Primary ohms and secondary ohms all check out ok. No leak between primary and secondary. Today was unusually hot in the plant.

We are using Bussman Low Peak Class CC fuses ( time delayed ) to protect both transformers, part # LP-CC-XX xx=current . Am I using the right type of fuse for thes transformers.

I think the NEC allows up to 225% of full load when both the primary and secondary are fused. (Sorry, not familiar with Canadian regulations).

I use LP-CC and FNM class fuses for transformers all the time without issues.

Is the 500VAC line something recent (power company stepped up in anticipation for summer AC usage?) or has it been that high for the life of the systems?

Tharon said:

I think the NEC allows up to 225% of full load when both the primary and secondary are fused. (Sorry, not familiar with Canadian regulations).

I use LP-CC and FNM class fuses for transformers all the time without issues.

Is the 500VAC line something recent (power company stepped up in anticipation for summer AC usage?) or has it been that high for the life of the systems?

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I am actually on site in Maryland. This is an area hit hard economically and a lot of industries have closed up. This could be the reason the 460/480 is so high in this place, the local utilies have probably adjusted the main feed into the area based all those industries up and running, so the actual load is low.

This place is so far from anywhere, Camdridge MD, that the closest electrical distributor is a 1 drive away in any direction. I plan on upping the primary fusing on the transformer that blew the 7.5amp fuses to 10a to see what happens. But getting Bussman CC fuses is a Monday thing right now.

Looking at the Bussman Data Sheets, they don't mention Transformers for LP-CC Fuses, but they do for FNQ-R CC Fuses. It may be the sensitivity to Inrush Current.

Do the Control Fuses open immediately, or after a number of seconds?

Stu....

I do use the FNQ-R style as well (actually the preferred ones to use). I just know we have many that have LP-CCs. Probably because someone replaced a fuse with one that had the same number on it one night after fixing a problem. (I probably did that myself at 2am a few years back...)

I think the inrush is definitely what's blowing the fuses, even though they are time-delay. NEC allows 250% for primary fuses, when the secondary is fused. Here's some info from Sola...

http://www.emersonindustrial.com/en..._temp_rise_ventilated_xformer/overcurrent.pdf

Assuming your secondary is fused, it looks like you can use 7.5A for the 1.5kVA and 15A for the 3kVA (based on page 2 of that PDF).



-Eric

Eric Nelson said:

I think the inrush is definitely what's blowing the fuses, even though they are time-delay. NEC allows 250% for primary fuses, when the secondary is fused. Here's some info from Sola...

http://www.emersonindustrial.com/en..._temp_rise_ventilated_xformer/overcurrent.pdf

Assuming your secondary is fused, it looks like you can use 7.5A for the 1.5kVA and 15A for the 3kVA (based on page 2 of that PDF).



-Eric

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Thanks,

yes secondary is fused, our panels would not pass code without this. I have suspected the temperature/ fuse rating is behind what is going on. All week no issues, then on the hottest day of the week fuses pop.

curlyandshemp said:

Thanks,

yes secondary is fused, our panels would not pass code without this. I have suspected the temperature/ fuse rating is behind what is going on. All week no issues, then on the hottest day of the week fuses pop.

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If they are opening after some time, that makes sense, as the Time Delay part of the Dual Element Fuses is normally Thermal.

Stu....

sthompson said:

Looking at the Bussman Data Sheets, they don't mention Transformers for LP-CC Fuses, but they do for FNQ-R CC Fuses. It may be the sensitivity to Inrush Current.

Do the Control Fuses open immediately, or after a number of seconds?

Stu....

Click to expand...

What got me was a transfomer working all week suddenly popping fuses on the primary side after the MCC was switched OFF then back to ON. After replacing fuses and same problem occurred , troubleshooting began. Powering up MCC with no load on transformer did not pop fuses. Adding load one power supply at a did not pop fuses either. Switch on MCC with all power supplies on, pops primary fuses. Only way to prevent this is to up calculated 3amp primary fuse to 6amp fuse and transformer no longer pops fuse on power up.

Unfortunatlety for the other transformer I have no 15amp fuses on site and need to wait till Monday and a 2 hour return drive to a distributor to pick up fuses.

I think this interwebs is just more than a fad. Doing research based on answers from this forum and keeping our in house master electrician and electrical engineer away from the bar on a Friday night, we have determined :
1 - Fuses should be FNQR not the LP
2 - ratings are half what they should be

thanks to those who answered.

I would check the connections on the load side of the MCC disconnect
for looseness or arcing. Check for voltage drop (line side to load
side) across each pole of the dissconnect when closed. If you have
6 amp fuses opening on the primary of a transformer with no load
on the secondary, your transformer is shorted or grounded internally
or, something is causing primary current to increase which is not
related to the transformer primary. Arcing at the dissconnect or
branch circuit feeder may not be sufficient to trip ground fault
and short circuit protection, but cause instantaneous currents higher
than tranformer primary fusing on power up. Most likley one of the
three phases is common to both transformer primary connections. I
would check the integrity of connections on this phase all the way
back to its source.

I think you should size the fuses in the primary for short circuit protection and the secondary for overcurrent protection. That should stop the nuisance fuse blowing and you should be OK for both types of faults.

curlyandshemp said:

I think this interwebs is just more than a fad. Doing research based on answers from this forum and keeping our in house master electrician and electrical engineer away from the bar on a Friday night, we have determined :
1 - Fuses should be FNQR not the LP
2 - ratings are half what they should be

thanks to those who answered.

Click to expand...

As a follow up, Had a some luck (FINALLY) in my favour on this job. In trunk of my car found a 3 pole fuse holder from who knows where. Inside the fuse holder were 3 x 15amp CCMR fuses. Put two of these fuses into the primary of the 3KVA transformer, and the fuses did not blow!

Have a 3 hour round trip drive Monday to Dover Delaware to pick up the only FNQ fuses in this area.

curlyandshemp said:

In trunk of my car found a 3 pole fuse holder from who knows where. Inside the fuse holder were 3 x 15amp CCMR fuses.

Click to expand...

That's why I never clean out my car. It's my portable 'junk drawer'...

Good to hear you're powered up again!...



-Eric