Solar DC question

Greenautomator

New User
Location
United States
Hello all,
We have a 175 Kw 1500Vdc string inverter. It has 12 mppt channels with 2 inputs each. Each channel (you can have 1 or 2 strings per "chanel") has 1 DC contactor for the positive and 2 DC contactors for the negatives.
Does anyone know the specific UL requirements for this and where to find it? The requirement being referred to, I believe is breaking the negative twice in a DC circuit. I just cannot find the reference for that.

I tried to attach a picture but was unable. Basically just shows the positive having one contactor and the negative having two.

Thank you!
 

Besoeker3

Senior Member
Location
UK
Hello all,
We have a 175 Kw 1500Vdc string inverter. It has 12 mppt channels with 2 inputs each. Each channel (you can have 1 or 2 strings per "chanel") has 1 DC contactor for the positive and 2 DC contactors for the negatives.
Does anyone know the specific UL requirements for this and where to find it? The requirement being referred to, I believe is breaking the negative twice in a DC circuit. I just cannot find the reference for that.

I tried to attach a picture but was unable. Basically just shows the positive having one contactor and the negative having two.

Thank you!
It might be a three-pole contactor. They are more common than two-pole. It's something we have done. To break DC you need quite a lot more clearance than on AC. Placing the contacts in series is one way of doing this.

I'm from across the pond. We don't normally have to be UL compliant so I can't comment on that.
 

pv_n00b

Senior Member
Location
CA, USA
There is no UL requirement to have two poles in a DC circuit but many manufacturers have to have two poles to get the disconnect to pass the UL listing for use in DC circuits. It's just harder to break the arc in a DC circuit and it's cheaper to have two poles than to have an expensive arc extinguishing system and one pole.
 

GoldDigger

Moderator
Staff member
There is no UL requirement to have two poles in a DC circuit but many manufacturers have to have two poles to get the disconnect to pass the UL listing for use in DC circuits. It's just harder to break the arc in a DC circuit and it's cheaper to have two poles than to have an expensive arc extinguishing system and one pole.
And in some cases getting the required DC voltage rating will require 3 contacts in series. No reason not to split them between + and - if you are not worried about manually breaking a ground fault. No special reason (IMHO) to put the two contacts in the - side instead of the + side. I assume this is an ungrounded array?
 

Besoeker3

Senior Member
Location
UK
And in some cases getting the required DC voltage rating will require 3 contacts in series. No reason not to split them between + and - if you are not worried about manually breaking a ground fault. No special reason (IMHO) to put the two contacts in the - side instead of the + side. I assume this is an ungrounded array?
You can get two-pole DC rated contactors. We made a lot of DC variable speed drives, mainly for paper making machines. Often DC contactors were specified by the old school customers. These DC contactors were "bar and shaft". Cumbersome beasts. And very expensive.

https://www.schneider-electric.co.uk/en/product-range/667-tesys-b/
 
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