Service disconnect

Purr24

Member
Location
Pennsylvania
Occupation
Electrician
Have you ever seen a 3 Phase service disconnect have this jumper installed? Brand new out of the box?


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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Yes, as well as some equipment that can be used on 1ph or 3ph systems. It's done when 1ph/3ph equipment is wired for 1ph.

It effectively places the two poles in series, which makes it withstand arcing better, because the switch is effectively opening faster.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I've seen it instructed most often with industrial (i.e., fork-lift) battery chargers (which are most definitely A.C.)
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Yes, as well as some equipment that can be used on 1ph or 3ph systems. It's done when 1ph/3ph equipment is wired for 1ph.

It effectively places the two poles in series, which makes it withstand arcing better, because the switch is effectively opening faster.
Why is the top part of the jumper on the "B" leg and the other end on the "A" leg?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Similar thread from a few years ago.

 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Why is the top part of the jumper on the "B" leg and the other end on the "A" leg?
Because you land incoming L1 on A and the outgoing L1 on B, making them in series. L2 in and out on C as usual.

Overall, for a line-to-line load, all three switch poles are effectively in series, effectively increasing opening speed.

I've seen 3p breakers wired like this for 1ph operation on convertible equipment, like the battery chargers above.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I understand the use for DC, having two blades in series helps break the arc, but I am not clear why or when you would use this method for AC when the switch is rated for the load?
It allows the device to last longer, and possibly allows for use at a greater ampacity.

It also works with relay contacts because it reduces pitting and increases contact lifespan.

Wiring contacts in series is more effective at this than wiring them in parallel.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Because you land incoming L1 on A and the outgoing L1 on B, making them in series. L2 in and out on C as usual.

Overall, for a line-to-line load, all three switch poles are effectively in series, effectively increasing opening speed.

I've seen 3p breakers wired like this for 1ph operation on convertible equipment, like the battery chargers above.
So "B" is only energized when the lever is up/closed. So this is for single ø ?
 

GoldDigger

Moderator
Staff member
I am pretty sure that if you cross reference the manufacturer part number ordered, you will see it described as a single phase disconnect rather than a 3-phase disconnect.
3-pole but not 3-phase.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I am pretty sure that if you cross reference the manufacturer part number ordered, you will see it described as a single phase disconnect rather than a 3-phase disconnect.
3-pole but not 3-phase.
I agree. For example you can get a 3 pole safety switch that has a fuse block between the first and second pole with all 3 pole in series and a separate neutral bar. You wire line in on the line side of pole 1 and load connects to the load side of pole 3. The cool thing is then you have a fuse that is dead on both ends with the switch open. And yes, it is sold as a single pole switch as you mention.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I understand the use for DC, having two blades in series helps break the arc, but I am not clear why or when you would use this method for AC when the switch is rated for the load?
You wouldn't. This is a "Solar String" disconnect, which is a standard AC disconnect switch adapted for use at 600VDC, which REQUIRES the series sets of contacts in order to get the necessary arc separation distance. There may also be magnets in the switch body used as "magnetic blow outs" for the arcs. You pay extra for all of those features in order to attain the 600VDC rating, so although you CAN use it for single phase AC, you would have wasted money buying it for that.
 

Purr24

Member
Location
Pennsylvania
Occupation
Electrician
I am using this disconnect for a single phase 208v AC service disconnect. I took the jumper out and landed my wires on line side L1 and L3 left nothing landed on L2. Load side the same. Is this ok?

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I am using this disconnect for a single phase 208v AC service disconnect. I took the jumper out and landed my wires on line side L1 and L3 left nothing landed on L2. Load side the same. Is this ok?
Technically speaking, it's a no-no because you modified an assembly.

The question is whether the modification reduced the switch's rating.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I am using this disconnect for a single phase 208v AC service disconnect. I took the jumper out and landed my wires on line side L1 and L3 left nothing landed on L2. Load side the same. Is this ok?

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You could have left the jumper and wired accordingly.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I am using this disconnect for a single phase 208v AC service disconnect. I took the jumper out and landed my wires on line side L1 and L3 left nothing landed on L2. Load side the same. Is this ok?

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If you look closely, the fuse holders are only on L2 and L3, so by using L1 and L3 you are not going to have fusing on one of your conductors, which would be a violation right there.

But also, even if the label said it was OK to use for AC, you said "Service Disconnect", which would also require a label saying it is "Suitable for Use as Service Equipment (SUSE), and that would ralso equire a Neutral /GEC Lug. I don't see that in there and I would not expect it to be there as a Solar String disconnect switch. That would make it a violation too.
 
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