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#88806 - 07/29/04 12:17 PM Extra load on transformer primary.  
rad74ss  Offline
Member
Joined: Apr 2004
Posts: 156
Pryor, OK USA
I have a problem at work. I have been told to tie in our phase monitor (460/3/60) between the transformer primary and the primary fuses. That just seems wrong to me. There are set ways to determine the primary fuses of a transformer. That does not include an additional load between the transformer and the primary fuse.

The phase monitor cut sheet says to use 2A max fuses type KTK-2 or equivalent. The draw is in the milliamps for this device. I just have to take issue with tying into the transformers primary fuses.

I was also told it was to save the cost of putting in seperate fuses for the phase monitor itself.

Am I being overly concerned about the welfare of the transformer, or is there a prohibition against tying into a transformer like that?

Edited for the following memo and censored to protect the innocent and the guilty. Now to see if it works!

xxx,

I have a few concerns about phase monitor fusing that I wanted to raise before the manufacturing instructions are changed.

The cut-sheet on our standard phase monitor calls for 2 amp maximum fast blow fuses. They are Bussman KTK-2 fuses that fit in our standard C02605-01 fuse block. They are $2.08 each per xxxx at xxxx. That would be a total of $6.24 for the phase monitor fuses. Our current kit is $6.09 for the fuses.

Our standard fused phase monitor kit uses 5 amp time delay fuses. If we tie into the transformer or motor fuses, the fuses are going to be time delay as well.

From my conversation with SSAC the time delay fuses will not protect the phase monitor during short circuit conditions. That is why they recommend the fast blow fuses.

Our list price for the phase monitor option is $XXX(Alot!) for XXXXXX equipment with an actual cost of around $XXX. (Not so much) The phase monitor is around $XX. The standard fuse kit is just over $8 with fuses and block. The cost for the phase monitor option including switch, relay, boot, and fuses is around $XX.

I think it would be economically feasible to provide the proper fast blow fuses with each phase monitor and still come in under our standard pricing. The fuses we currently tie in to do not provide the protection that the phase monitor requires. The fuses are not very expensive and can easily be covered by the amount we charge for them and the amount of our cost for the option. We already cost the option with enough money to cover fusing every phase monitor.

Another issue is tying into the transformer primary fuses. The fuses on the primary can range from 1.6 amps to 12 amps for our most commonly used transformers. The fuses will not protect the phase monitor due to size and the time delay action of the fuse.

In summation we could fuse each phase monitor with the proper type fusing and still come in at the same price we charge for just the phase monitor option. We would be providing the maximum protection for the phase monitor without it being a cost issue, being that the option has enough money to cover standard fusing.

Thank you for your attention.

Robert Dollarhide

08/03/04




[This message has been edited by rad74ss (edited 08-03-2004).]


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#88807 - 07/29/04 06:44 PM Re: Extra load on transformer primary.  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
You'll hear some bitching and sniveling, but insist on some Bussmann KQV cable limiters and use 12AWG SIS-VW1/XHHW-2 switchboard wire to connect your phase monitor. That’s exactly what they’re made for—bus taps for this kind of stuff. [The #12 might as well be RDX det cord if faulted.]

The need is switchboard-burndown short-circuit protection for the protective-relay tap—not overload protection. This gets "unscheduled-production-outage explosive" if omitted.

They were about $12 each last time I got ‘em. www.bussmann.com/library/bifs/1042.pdf Reference NEC 240-21(c) (and common sense.)




[This message has been edited by Bjarney (edited 07-29-2004).]



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