8 pole dc isolator for 2 strings of pv ?

[vitoboy]

hi all
ima spark not mcs but have done some bits of solar installs for others.
my question is this...
i have 2 rows of pannels 15 in all (7 in one eight in the other) and they are wired back in 2 pairs.
ie at the dc isolator i have 2 lives and two neutrals.
ive been told that i need an 8 pole isolator rarther than a 4 pole,so each line is in effect isolated twice.
ive been looking around and cant find any reference to this..only the need for a double pole isolator ie both lives and neutrals are isolated which would mean only a four pole isolator...?

many thanks vitoboy

 

[yellowvanman]

If you look inside some DC isolators, its effectively a 4 pole isolator and they have put links in to give 2 breaks per pole, making it a 2 pole isolator, there are others on the market (IMO?) that are truly 4 pole and don't need the links, I think because they have a larger gap between contacts.

 

[vitoboy]

ta yellowvanman
but do you know where i can find the reg that requires a double isolation or a larger contact gap..
i thought that if applicable it would be in g83....
but theres nothing i can find

ta vitoboy

 

[yellowvanman]

I don't know where its specified, only that there is a difference between ac and dc isolators, because of the nature of having to break currents for ac and dc.

Obviously where an isolator has links put in by the manufacturer then thats to comply with whatever requirements they have to meet! So whilst its actually 4 pole it sold as a 2 pole.

 

[SibertSolar]

OP - I'm not aware of any regulation that requires you put 2 DC strings through an 8-pole DC disconnector. I am aware though of 8-pole units having to series-link each pair of poles to achieve a 4-pole output rated adequately (as in, say, 16A @ 450VDC). However, these units are sometimes simply AC isolators in their design and functionality.

Yellowvanman is right on the money with the IMO disconnectors - they are designed to load-break DC current/voltage (long arc-cooling chambers, knife-edge self-cleaning contacts, maximum 5ms make/break etc). A 4-pole unit (same size/footprint as 2-pole) can indeed be used to disconnect 2 separate strings. As standard, without linking any poles together, they are available in ratings such as 16A @ 800V or 25A @ 600V. Only if 1000V ratings are required would you need to series-link any poles together.

If you want to PM me your phone number, I'll give you a call in the morning as we have stock of a suitable solution for you. Or call me on 0777 5427243.

Andy

 

[moggy1968]

I have used the IMO 4 pole isolators from Sibberts and they are excellent and save having to use 2 DC isolators. Do be sure to check the wiring diagram and buzz them through if your not sure so you know what links to what as it is different to what you have probably seen before. It's all pretty simple though, they are labelled.
For DC you need a 4mm seperation of the poles to allow for the greater arcing of DC. The original DC isolators were basically modified AC units as Andy suggests so the only way to build in adequate seperation was using links. It was a bit Heath Robinson TBH and I prefer the IMO ones as they are clearly designed specifically for the job in hand.

 

[Jon.berry]

The issue with IMO is that there isolators is that although knife contacts are better for DC there switches have a snap action latch with a padlockable handle which cannot pass the IEC standard for switch-disconnectors and should only be used in north America or anywhere UL is accepted.

 

[SibertSolar]

Interesting comment.....which IEC standard are you referring to please?

 

[Jon.berry]

Bs en 60947-3

 

[SibertSolar]

Thanks Jon. Strange that you reference this standard as it "normally" is used in reference to AC category switching. However, as far as I am aware (and I'm checking the details) the IMO range of SI disconnectors is compliant with BS EN 60947-3 but I can't see where there is any reference to the actuation of the rotary handle or switching mechanism within this standard.

Is it AC isolators you are referring to perhaps, in relation to this standard? Do you happen to work for Kraus & Naimer by any chance?

 

[Jon.berry]

BS EN 60947-3 is intended for AC and DC it has no preference and that is indicated under the scope of the standard. You need to look in part 8.2.5 of the standard all snaplatch switches with padlock devices fail this but it is only a IEC requirement, not UL. Benedikt & Jäger manufacture the switches and IMO sell them under licence rebranded for themselves. In itself it is a good product apart from having a padlocking handle... You are correct K&N technical engineer. K&N themselves have knife contact switches but the same as other manufacturers (Santon, ABB, Socomec to name a few) there are always issues with selling products with padlocking handles and snap latches within the IEC. Generally this is why most manufacturers have gone with standard double break contact systems and invested in testing the product for DC.

It is nothing new, K&N and other manufacturers have been selling DC switches for years before PV came along.

Although i see you sell IMO & i work for K&N so maybe we will be a little byist in our opinions! :hair:

 

[SibertSolar]

Thanks Jon, I did wonder why you were drawing attention to the "snap-lock" mechanism....

I am well aware of the Benedikt and Jager/IMO relationship and also well aware of the supposed "concern" from K&N over the operator independent spring/latch mechanism of the IMO/Benedikt but isn't that simply your only line of "attack" by virtue of the fact that you are, in essence, comparing modified AC isolators with true DC disconnectors? Your comments seem to suggest that your KG range of "DC switches" fall under the same design umbrella as Santon and that the only differential between that group of products and the Benedikt design is the action of the switching mechanism? I agree that the K&N and ABB units are very similar (modified AC isolators) but I would disagree about the Santon DC switches. They are not the same type of unit as far as I am aware....

Just because arcing/welding of contacts is a concern when using AC isolators to load-break DC doesn't mean that it is a concern with the IMO units. The design of the IMO units incorporates several features that help eliminate not only the generation of arcs but also the impact such arcing might have on the contact face of the terminals/contacts (no impact at all in IMO's case).

What design features do the KG range of K&N units contain to minimise/prevent/manage DC arc generation?

Also, as you are a direct representative of K&N, can I ask if you can advise what "rapid handle operation" means on your spec sheets please in relation to the DC category switching ratings? I've never yet got a satisfactory answer on that one.....

I think we'll struggle not to be a little biased in our opinions, as you rightly mentioned! All good harmless banter/discussion though I'm sure.....

Andy

 

[Jon.berry]

Andy,

Maybe it would seem that way but my experience in the switchgear industry and testing of product in compliance with the standards gives me a unique view point which other mainly users may not be aware of. You will find alot of switchgear companies don't attack... and it was not my intention. As i said Benedikt have a good product, just not accepable under IEC with a padlock handle. It would be the same if K&N were to market their knife contact/snap latch switch in an enclosure with a padlock handle. we ourselves use ABB switches and others, you will find that Moeller buy our switches and ABB etc. Switchgear in effect is like a family generally good companies use the best products from their competitors aswell as their own.


I am not here in a K&N compacity but am happy to answer your question.


1. Arcing and welding happens on all switch types AC & DC regardless of the contact system and other design advantages. It is how quickly you can build up resistance to blow the arc out that helps you acheive higher ratings for the switchgear. It is not just how much voltage and current that determines the ratings but the speed of operation, thermal rating, contact gap, switch material, contact type, number of operations, utilization category for the load type etc etc...

2. There is not such thing as a true DC isolator.

3. I think it is better explained as this;

The KG switch range is like say a ford focus, the car was designed and built for safety and can be fitted with a diesel (say DC if you like) or petrol (say AC if you like) engine.

The KG range was design with the switch-disconnector standards IEC / BS EN 60947-1 & 3 in mind, and has been tested for various different utilization categories for AC and DC use (the same as any other manufacturer, but maybe due to design of a certain switch type it is advantageous to market the product for one voltage set instead of both).

If say 2 cars were tested for a 5 star ncap rating and both compiled (say in our example both would isolate 11A @ 500V, DC-21B) then that is it. Regardless of anything else both would isolate or say stop safely in a test.


4. "rapid handle operation" - why not? anyone working with DC knows speed is a major point in building up resistance to blow the arc out. Or combine this with arc shutes or magnets or other methods (the common method being series connecting of contacts) and you have a very good means of blowing out the arc. Really this is such a minor point and more of a helpful note. It doesn't have any impact on safety.

Regards

Jon

 

[SibertSolar]

Hi Jon,

Thanks for the input, much appreciated. I hear what you are saying about the nature of the tests/results for DC category switching and how if 2 different units pass the tests, regardless of the their inherent design differences, then they are both compliant. The point about the IMO units, in my opinion, is that they have arc dissipation/suppression features built into the units. This extends the life of the unit and, combined with other design features, minimises the impact of arcing on the contacts.

At the end of the day, the K&N and IMO/Benedikt units are different beasts in terms of their design - the costs reflect this. Customers are encouraged not to compare "apples with pears" so to speak. I accept that the K&N units meet the utilisation category requirements but I fail to see why there is this ambiguous link with "rapid handle operation". If it has no impact on safety then why mention it at all in relation to the utilisation category?

Regarding the IEC standard and snap-action contacts, I'm not entirely sure about that yet but I'm looking into it so thanks for flagging that up,

Thanks

Andy

 

[Jon.berry]

Andy,

As far as i'm aware K&N have dropped that from its literature just due to the fact its not required, it bears no impact and is not part of the test procedure for IEC / BS EN 60947-3. Just a translation issue as far as i'm aware from the initial German catalogues.

Have a look at parts 7.2.4.3 of 60947-3 & 1. Also table 9 of 60947-3.

Regards

Jon