Control transformer protection

[itsmoked]

I am building a control panel in a large disconnect enclosure.
It has a PLC in it, two 1HP motor starters that are overload protected and are individually magnetic breaker-ed. Thing runs on (USA)480-3Ph. The whole thing is fed from a breaker panel which provides the thermal protection aspect.

I have to include a 1kVA control transformer. I was about to spec out a fuse block pair when I noticed no protection for the existing system except for a 10A breaker on the 120Vac side of the control transformer. Should I bother with the fuse block or leave primary protection to that remote breaker panel?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi keith.
I don't like adding transformers that big inside a control panel. I try to stay away from ventilating fans bringing contaminants into the panel and I have seen too many existing panels were, instead of replacing the fan filter when things got hot, someone chucked it. I sometimes mount transformers larger than 500 VA on the outside of the panel to keep the heat outside.
Respectfully

 

[itsmoked]

Hi waross; I would agree with that logic. In this case the actual control system is probably 100W max the 1kVA is to feed a duplex "courtesy outlet" to run some "door open" device once a decade... So it won't normally be dumping a bunch of heat but rather just keeping the moisture out.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

In that case I would probably put it inside also.
Respectfully.
Hi Keith

 

[dpc]

I'd put the primary fuses in. It's asking a lot of the breaker upstream to detect that the CPT 480V winding shorted out a few turns. The fuses will improve your odds a little, anyway. These little guys have been known to go up in flames, as I'm sure you know.

 

[itsmoked]

Thanks dpc; I will proceed with fuses. Besides if I look at Grainer the Square D 1kVA costs $278.

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611760969

If I get the exact same transformer but with a primary/secondary triple fuse block mounted on it, it only costs $226. Add a fuseblock deduct 50 bucks.. go figure!

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611773146

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith
It might be woorth a call to the local Electrical wholesalers like Graybar. They may have the same parts at better prices.
Respectfully

 

[jraef]

Keith,
Pricing is all about volume. Most people buy them with the fuse blocks attached now. BTW, make sure you use time delay fuses.

Also BTW, CBs are OK by some rules, but IMHO a bad chice for primary protection. Besides, UL requires secondary protection, whereas in the past the NEC allowed primary only in some cases. You are taking the right appraoach now.

Also BTW²,
If you are going to have this panel inspected by UL, watch out for another trap that has popped up recently. Leviton and others are selling a really nice panel mount duplex receptacle that has it's own box and clips to DIN rail. UL inspectors have been bouncing them for applications such as you just described because technically they are not UL listed to have ANYTHING plugged in to them that is external of the panel! I kid you not. I guess that when they UL listed them, the intent was for internal use on systems that had "wall wart" PSUs, so UL listed them ONLY for that use. If you plug a trouble light or drill motor into them, they are void.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[fangas]

I've done the utilty/control transformer thing a bunch. In fact, just about every control system I've done in past 15 years has had them.

I'd hang a 1kVA distribution type outside. Fuse the Primary side. Separate the sec. Use CB's for them. 1 for the utility load & 1 for the control load.

The reason, it's going to get loaded with anything and everything. If you fuse the sec, someones going to do lot's of fuse replacement. If you seperate the sec, then when the utility load (Fan, heater, tool, ???) overloads, the machine will at least keep functioning.

I wouldn't believe that once a decade thing. A main't guy or operator looking for some power is gonna take it from the most convenient location they are aware of.

Oh yeah, I also usually hang an outdoor type recepticle box outside of the enclosure. Put a brushed aluminum plate on it, it'll add a touch of class

Ed

 

[itsmoked]

fangas; Very nice points. I shall contemplate the exterior plug idea.. It does invite someone to use it, possibly permanently, which could bring me into the 'hot transformer in a box scenario' whereas if they have to open the door to use it then at least the air will move. I have already procured the transformer and it is not an externally mountable one. Still a nice clean idea...

It is primary fused and secondary fused(12A) and will have breakers for the control and for the outlet.(Now that you have suggested it!)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com