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250 Volt Equip/480 Volt Equip #54100 07/17/05 05:20 PM
Joined: Jun 2005
Posts: 124
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poorboy Offline OP
Member
Luckily I noticed some of the 100 Amp fusible disconnects the supply house gave me for my 480 volt service were General Duty, 250 V rated. Could have easily gotten a bunch of stuff all put together and had to take it out.

I remember seeing a 250 volt panel full of 250 volt breakers running a 480 volt log home cutting line that had been in service for years.

I have a feeling this kind of thing occurs from time to time, and am wondering what the greatest dangers of it are. Would a dead short be more likely to cause parts to fly? Is it mostly to handle the fault situations that the construction is different?

Don't get me wrong, I am not looking to cut corners, just curious what some of the possibilities are.

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Re: 250 Volt Equip/480 Volt Equip #54101 07/17/05 05:29 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
I was once touring the back spaces of one of Reno's better casinos, when I saw 250 rated disconnects on 480 v equipment.

There are two areas where the non-rated disconnects are most likely to fail:
- In time of need, the disconnect will fail to interrupt the current, as the current will arc across the gap; and,

- A small amount of dirt, or damage, will lead to an internal line-to-line arc.

There's areason the 600 v. ones are physically larger!

Re: 250 Volt Equip/480 Volt Equip #54102 07/17/05 06:11 PM
Joined: Oct 2000
Posts: 2,720
Scott35 Offline
Broom Pusher and
Member
#1 reason:

Arc across blown fuse (or open contacts of a Circuit Breaker):
240 VAC OCPDs are likely not going to have enough of a gap, and Arc will not be extinguished, when opened under load on a >240 VAC Circuit.

#2 Reason:

Throw open a 240 VAC EXO Switch (Disconnect), and a 600 VAC EXO Switch - both being the same duty rating (General or Heavy).
Notice the amount of travel for the handle, plus the intensity of the "THUNK!!!" which results from opening the Switch.
Also notice the amount of travel for the "Knife-Blades" inside the Switch.

Opening the 240 VAC Switch under load, when connected to a 480 VAC Circuit, will likely produce an Arc between the contact points, which will continue to run for a long time - until something stops it (like the Fire Department, etc.).

The Arc containment and extinguishment considerations are the primary reasoning behind having Equipment ratings for 240 VAC and less, or upto 600 VAC.

You could use 600 VAC rated Equipment on a 240 VAC System, and have no undesirable results.
The same cannot be said of the opposite - using 240 VAC EQuipment on a 480 VAC or 600 VAC System will hide the disasterous effects, until the time comes to unmask its effects!... either a Fault or Overload scenario.

Now, there are "Dual Listings" on things, which may apply to the quoted text below:

Quote

I remember seeing a 250 volt panel full of 250 volt breakers running a 480 volt log home cutting line that had been in service for years.


Have seen Panelboards with devices which appeared to be "Too Small To Be >240 VAC Rated", but in actuality were the correct rating.
Square D's type "EHB" frames is a good example.

Have also seen the misapplication mentioned by Poorboy - and that is really scarry!
Yes, it has been working this way for years, and it is difficult to explain what the disasterous results will be ... someday, which makes it even more frustrating to have someone justify their "Who Cares?" attitude!
Been there too, Poorboy!

Quote

I have a feeling this kind of thing occurs from time to time, and am wondering what the greatest dangers of it are. Would a dead short be more likely to cause parts to fly? Is it mostly to handle the fault situations that the construction is different?



You are correct in feeling the reasons behind why there is 240 VAC rated Equipment and 600 VAC rated Equipment!

Scott35

edited a stupid grammatical blunder! [Linked Image]
hope there's not any more! [Linked Image]

[This message has been edited by Scott35 (edited 07-17-2005).]


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: 250 Volt Equip/480 Volt Equip #54103 07/17/05 06:49 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Recently got involved with a new account, one of the first projects was going to require a new 480 volt 400 amp bucket in their service gear.

We quickly found the 1200 amp service gear was indeed running at 480 but all the Westinghouse Tags say 208Y/120. It's been that way 20+ years. [Linked Image]

It appears it may be just mislabeled as it uses 600 volt fuses. The PM is researching it.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

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