DC Disconnect

[electro7]

Hi,

I am looking at installing a Square D 3 pole non-fused 600VAC/600VDC safety switch at a ground mount array. I will be installing 3 strings of 12 modules with a Impp of 8.15A and Isc of 8.66A. I was wondering if a single 30amp safety switch would suffice to disconnect the positive of each string? Do I use the Impp or Isc rating of the module to calculate this? And do I need to multiply it by 1.25? Would I need to step up to a 60amp? Quite a difference in price.

Thanks

 

[SolarPro]

Here's a link to the latest revision I can find of the relevant Square D's data bulletin. Note that this use is not UL listed, but is instead "self-certified."

Here's a similar product from Siemens that is listed for this application. (However, I've heard that the basis for the listing is being disputed.)

 

[GoldDigger]

Here's a link to the latest revision I can find of the relevant Square D's data bulletin. Note that this use is not UL listed, but is instead "self-certified."

Here's a similar product from Siemens that is listed for this application. (However, I've heard that the basis for the listing is being disputed.)

Note that the Square D product is UL listed when wired so that each circuit is interrupted by two or more switch poles in series. It is the single contact per circuit use that is self certified, and I think that this is the configuration the OP was looking at. It is not 100% clear from the post, since "one switch" could be either on assembly or one pole of a single switch.

The Siemens product, on the other hand, incorporates specific features (permanent magnets) to assist in breaking the DC arc, for whatever that may be worth. I believe that may make the design polarity sensitive.

 

[jaggedben]

Hi,

I am looking at installing a Square D 3 pole non-fused 600VAC/600VDC safety switch at a ground mount array. I will be installing 3 strings of 12 modules with a Impp of 8.15A and Isc of 8.66A. I was wondering if a single 30amp safety switch would suffice to disconnect the positive of each string? Do I use the Impp or Isc rating of the module to calculate this? And do I need to multiply it by 1.25? Would I need to step up to a 60amp? Quite a difference in price.

Thanks

The issue is typically the voltage, not the amperage, so stepping up to 60A will not by itself help you meet the proper ratings. As the other responses are addressing, usually you need to use two or three poles of a switch in series in order to have a valid 600volt DC rating. Many 600VDC disconnects are just lower voltage switches that come pre-wired this way.

If the Siemens product is too expensive or hard to obtain, you might compare the cost and hassle with using a combiner along with a higher amperage switch that is good for a single circuit.

You would use the Isc rating of the modules for all calculations, and you'd typically need to multiply it by 1.56. (that is, 1.25 x 1.25)

 

[Zee]

In addition to the excellent points made above, Google the following document:

(Square D)
Data Bulletin
3110DB0401R11/07


Direct Current and Photovoltaic Systems
Applying Heavy Duty Safety Switches (Fusible and Non-Fusible)
on DC and Photovoltaic Systems
Class Number 3110

It lays it all out clearly.

You are very right to ask. There is, in fact, a different amp rating of the disconnect whether one or all poles of a 3-pole Square D, dc disconnect is used!

Page 2: You can see that you can have 18 amps when multiplied by 1.25 and 1.25 again (1.56), so that is equivalent to 11.5 A Isc. Since you have 8.66 Amps, you are golden! You may connect s strings, one each to all 3 poles of a single, 30 A disco.

ps the "back and forth" wiring thru multiple poles of a disco is if you want amps above 11.5 and up to 30A.

pss i trust you are NOT using a transformerless or ungrounded inverter........

 

[SolarPro]

In the 2010 version of the data bulletin, which I link to above, the per-pole Isc-rating for the 30 A heavy-duty disco is 12.8 A for the fused version and 16 A for the unfused version. (Not sure when this data bulletin was originally published?or if it's even archived on the manufacturer's site any longer?but after 2007 it was revised in 2009 and then again in 2010.)

 

[electro7]

Thanks for your help on this one. I will take a look at that article. I saw a Siemens HF362RPV that looks good.

No in this case it is an inverter with a transformer. But for the future, is it okay to disconnect the positive and negative through the disconnect if using a transformerless inverter and an ungrounded array? I would be using two poles of a three pole disconnect.

 

[GoldDigger]

I do not see why not.
Although opening the disconnect in the presence of a fault to ground might cause the entire current interruption to be done by only one of the two contacts.

Tapatalk!

 

[ggunn]

Also of note is that the unfused Square D switches are rated at 100% amperage while the fused ones are 80%.

 

[Zee]

Thanks for your help on this one. I will take a look at that article. I saw a Siemens HF362RPV that looks good.

No in this case it is an inverter with a transformer. But for the future, is it okay to disconnect the positive and negative through the disconnect if using a transformerless inverter and an ungrounded array? I would be using two poles of a three pole disconnect.

1. Sure!

2. You got to what i meant about ungr. systems: make sure to interrupt + and - if ungrounded, that is all.

3. Yes, two poles of the Square D for + and - .....BUT NOT the SIEMENS! LAst time i checked, That is just positive. Unique. Some magnet deal.

 

[GoldDigger]

Since the poles of the Siemens unit are not connected to a bus, you still need to use two poles of the switch for an ungrounded array. You just have to bring in the - from the panel to the opposite side of the switch.
The result is that the current is flowing in the same direction across each of the switch contacts.
The reason it is polarity sensitive is that the direction the arc bends under the influence of the magnet is opposite for opposite current direction. But the arc chute or equivalent is only on one side.
You can make a symmetric switch by providing for the arc to go either way. It just takes more space and costs more.

Tapatalk!

 

[electro7]

Interesting, thanks!