DC Isolator or no DC Isolator that is the question

[netblindpaul]

Worcester,
Is this not why you must use suitably d.c. rated isolators?
The voltage will sustain the arc and the current will fuel it, thus it could be a big bang in the wrong situation!!!

 

[SibertSolar]

Very relevant and important messages being expressed by Worcester/Markc/JulianC/netblindpaul above. With AC isolators, even when series-linking multiple poles together to achieve a suitably-rated 2pole output, the fact remains that they are not typically designed to manage making/breaking DC currents/voltages.

Proper and correct DC isolation should not be a commercial decision - the important thing is to make sure that the product being used is designed for purpose.....

Andy

 

[Worcester]

@netblindpaul

Yes!

Sorry guys been at Intersolar.. out at 4am Thursday Back at 3 am Friday .... Need sleep!

Andy at Sibert can supply DC isolators (the one's in my timelapse install video) they're made by IMO and his prices are good,

Here's the technical info from IMO:

IMO Precision Controls - DC Isolators

(No big bang then!)

 

[Worcester]

The SMA ESS DC Disconnect is fully compliant as a DC Isolator, so an additional isolator is not required.

Ken Hobbs

Ken, I had a long chat with SMA technical about this and other matters the other day, especially regarding the situation when you have 50+ strings pumping out 800V (we have a few!), they were adamant that using the ESS as the primary DC isolator was not a good idea.

Yes it will do it, however the comment was that if you have a string (accidentally on hopes) connected with the wrong polarity, then if you disconnect it using the ESS you will get big sparks and a big bang, and the Inverter will go into maintenance mode and you wount be able to do anything with it until dark. You will also potentially invalidate the warranty.

Yes it will do it, however is not designed to cope with accidental incorrect wiring, so they recommended using seperate DC isolators.

 

[bhmike]

As soon as load is disconnected i.e. ESS is removed, simply unplug DC side from inverter

 

[Worcester]

@bhmike, the problem is in how the DC circuit is broken.

Some of the Isolators sold on the market as DC isolators are actually modifed AC isolators (read the earlier posts) and may not effectively isolate the DC after a period of use.

Switching DC is very different from switching AC.

There are also bigger issues also , you now have a pair of dangling cables that are 'live DC' (during daylight hours )

if you read the earlier posts you'll see the whole discussion.

 

[JH69]

Sorry to bring up an old thread, but this discussion is relevant to my situation.

We’ve just had a new PV system installed with 15 x 265W panels and a SB4000TL inverter. The installer has not fitted any additional DC isolators on the basis that the DC breaker is in the SMA ESS switch.

Reading this thread, it looks like opinions differ as to whether separate DC isolators are really necessary, but the general consensus appears to be that they are low cost parts so probably should be fitted.

The question is: is this something I should be concerned about?

Is it possible to retro-fit the DC isolators, and should I ask my installer to sort it out? (I’ve not been invoiced for the job yet).

I think if the truth be told, I would be happier if they were present, but I don’t want to cause too much hassle to my installer because for the most part they have done good a job.

 

[Gavin A]

for me it's about how you safely isolate the system at the point that the inverter needs replacing.

Though I suppose you could just connect both positive and negative connectors together, this assumes the replacement inverter uses the same connectors.

- - - Updated - - -

also I'm sure the ESS switch only breaks the positive side of the circuit.

 

[wstevew]

I have a 4000TL and have 2 DC Isolators.

 

[sedgy34]

Could be a problem for who is doin maintanance, as no local isolation.

 

[JH69]

Could be a problem for who is doin maintanance, as no local isolation.

Well, my first point of call for any maintenance will probably be the original installer. So if it's a problem for them, it's a problem of their own making!

The installer is bringing all the paperwork over later on this week, so I'll mention it to him then. But I know what he's going to say: SMA say it's ok. I discussed a few issues with him pre-install (such as cable spec, no of panels per string etc) he basically said that they work to the manufacturer's instructions. I actually phoned SMA myself this afternoon just to check with them and they basically said that you don't need a separate DC isolators if you have an ESS switch.

Personally, I would have thought it's sensible to fit them. But then what do I know?

 

[jason121]

MCS dont like the SMA idea, but SMA are a very big company and dont like being told what to do.

 

[sedgy34]

if you prefer a dc isolator to be fitted too ask your installer to fit one when he turns up its not much drama as hes returning anyway.

 

[Worcester]

Sensible yes, good practice, yes, in a multi string environment almost essential for maintenance and testing. Mandatory ... No as SMA have said it is adequate

Version 3 of the guide says ..

A switch disconnector shall be provided on the dc side of the inverter.
* The switch must isolate all live conductors (typically double pole to isolate PV array positive and negative conductors)
* The switch must be rated for dc operation at the system voltage maxima as calculated in section 2.1.2.
* The switch must be rated for dc operation at the system current maxima as calculated in section 2.1.2.
* The switch must be labelled as ‘PV array d.c. isolator’, with the ON and OFF positions clearly marked. Switch enclosures must also be labelled with ‘Danger - contains live parts during daylight’. All labels must be clear, easily visible, constructed and affixed to last and remain legible for as long as the enclosure.
* The switch should be located adjacent to, or integrated into the inverter.

So to the the letter it meets the final clause.

The guide goes on to say:

An additional d.c. switch may be specified for systems with long d.c. cable runs (typically at the point of cable entry into the building) – so as to provide a means of isolating the cable for safety reasons or maintenance works.

We ALWAYS fit DC isolators, because on an industrial sized installation the thought of pulling the ESS to break 17+kW of DC is horrendous.. On a recent job we used 4 pole isolators to work on two strings, however we are unlikely to be doing that again because when you make one pair live to test the string you've now got the other pair live also. also some of our strings run close to 1000V Voc and you get a b*****y great spark if you try to seperate those after doing an Isc test without first opening the isolator!

 

[Gavin A]

does it actually break both positive and negative though?

I'm sure it's only a double break in the positive circuit, so not a dual pole isolator.