DC Isolator or no DC Isolator that is the question

[Thegreenelec]

Does anyone know if you are using a SMA Inverter with its built in pull off DC Isolator, if you need a seperate DC Isolator fitting next to the inverter? I know in Europe they don't fit them. If we don't need to fit one, where does it say for sure in black and white?

Thanks

Paul

 

[BruceB]

Separate DC isolators are useful where you have more than one string coming into the inverter and to facilitate testing. But in my personal view, from a safety and operational perspective, they are not necessary where an inverter has a built in isolator. I think you can argue that the built in isolator satisfies 712.537.2.2.5. However I would acknowledge that it is a little bit grey. The only person likely to pick you up is a pedantic assessor.
Regards
Bruce

 

[netblindpaul]

I'm not familiar with the SMA inverter, however, how would you safely isolate the d.c. side in order to replace the inverter if you have no external d.c. isolation?

 

[SolarCity]

Like Netblindpaul says, the inverter isn't isolated unless a DC isolator is fitted. The in-built DC isolator effectively kills the DC connection the workings of the inverter but there still exists a dangerous voltage AT the inverter.

Doesn't seem like a corner worth cutting in my opinion.

 

[SolarCity]

you could unplug the strings from the inverter after you turned off the A.C Isolator and the D.C Built in isolator???

Good point. With Sunny Boys this is possible.

 

[Graeme Harrold]

Sunny boys come with MC-T4 connectors that can only be reached once the front panel is removed. The panel con only be removed once the DC Isolator (Solar Switch) is removed. Was told on my course, technically compliant, but always better to fit additional DC Isolators.

 

[Markc]

I see no great issue either way and yes agree a large grey area however could you be condemned for for fitting a DC isolator regardless? It's not the big money item after all.

 

[Worcester]

Nope, but when your've got 50 strings to wire up, the costs of the time and components do mount up, especially when the large inverters also have built in string fuses. Time for thought here.

I see that SMA have opened their UK office now, so time for a call.

 

[SolarCity]

With 50 strings, I agree that the cost does add up (although on a job with 50 strings I'd imagine it's a drop in the ocean) but we'd definitely still install the isolators.

What are your thoughts on testing? Without an isolator would this not make things tricky? Particularly Isc?

 

[Markc]

Nope, but when your've got 50 strings to wire up, the costs of the time and components do mount up, especially when the large inverters also have built in string fuses. Time for thought here.

I see that SMA have opened their UK office now, so time for a call.

I don't see any difference if it's one inverter, 50 inverters or 100 inverters. The actual cost of a DC isolator is minimal to the price of the job. Should such a small price item dictate weather it is used or not?

It's a grey area and a personal choice at the moment until the reg's catch up to the products. However there is a over ruling factor to any thing and thats good working practice.

I consider if in the future there's an inverter problem and the manufacture's appointed service agent is sent out. You know dame well that they will see an issue with the absence of a DC isolator regardless of if it's needed or not. How many times have we had this with boiler agents who don't under stand differences in wiring arrangements and just say it's wired wrong!

I personally will always fit. it's good practice. We use SMA and like the product, it's good stuff but regardless a DC isolator is fitted, every time.

Just my opinion.

 

[netblindpaul]

As markc says above, it the inverter instructions require d.c. isolator then it does not comply with regs unless this is fitted does it!

 

[JulianC]

Have always fitted separate DC isolators with SMA inverters and will continue to do so. As metioned above, the costs compared to the overall project cost are minimal and they facilitate much easier testing. Commissioned a 50kWp system this morning using 3 x SMA TriPowers all fitted with separate DC isolators to each string. Didn't add any significant time or money to the project and, as Markc says, no one will penalise you for doing a proper job.

 

[Worcester]

@Markc, @JulianC

We always put DC isolators in, just thought I cause a little debate

 

[Markc]

@Markc, @JulianC

We always put DC isolators in, just thought I cause a little debate

Your video confirms this also.

 

[Worcester]

I had a long chat with SMA today about this subject, and they are aware of a number of installations done this way (no DC isolators) - that's not to say it's right or they approve, just they know of some.

The problem is the fact that it is DC.

In THEORY, pulling the ESS switch on the Inverter will isolate the DC from the inverter, however, especially on their larger inverters this can only be done at night (little voltage / current) otherwise you could easily get massive arcing sparks and a bang! The inverter then goes into maintenance mode and you've probably lost 12 - 24 hours.

The above is why converted AC isolators don't work on DC, as the contacts break, (even in a DC isolator) you get arcing across the contacts, with AC this isn't a problem as the voltage goes to zero 100 times / second, with DC through an AC isolator the contacts can easily get welded, burnt, or even still be arcing, so although you think it's switched off, it isn't.

And live PV cables at up to 1000V and 15 amps isn't a good idea.