Sb3800

francie · Jan 12, 2011

They say there is no such thing as a stupid question and I have learnt so much from this forum, I hope you will bear with me. I have a SB3800 with an ESS switch. Do I need a DC lockable isolation switch? I am getting conflicting advice.

Also, the PV array test report asks for the protection device and I cannot get any protective device rating on the ESS switch.

JulianC · Jan 12, 2011

PV array test sheet protection device refers to string fuses or similar - leave blank if you are not fitting such devices.

As regards the ESS, I suspect you will continue to get conflicting advice are there does not appear to be a clear cut answer. On the one hand, the very action of pulling the ESS and unplugging the strings from the inverter isolates the inverter from the DC. On the other hand, it is a two-stage procedure to isolate in this manner (simply pulling the ESS still leaves your DC cables plugged in to the inverter, just not electrically connected), and it is not lockable. Unplug the DC cables and it leaves the ends dangling with only IP2X protection, so arguably not safe. I always fit a separate, lockable DC isolator and will continue to do so. Seems the right way to do it. It also allows you to easily carry out your DC testing.

In my (admitedly limited) experience, justifications for doing away with a separate DC isolator in favour of ones built into the inverter are to do with cost savings and nothing else. I had a similar discussion about this very issue last week with an ex-colleague, now employed as a PV engineer with a large roof tile manufacturer. He'd rang around a lot of his contacts (some proper old-school PV experts involved in writing the DTI guidelines, amongst others) and still couldn't get a definitive answer!!

Markc · Jan 13, 2011

I don't think you can be criticised for stick with best practice. We always fit DC isolators to any inverter that comes with built in isolation. Don't ask me if it's right it's just what we do.

francie · Jan 13, 2011

Thank you for advice. I also feel better about fitting DC isolators as well. As I have 2 strings going into the inverter, I assume I use one DC isolator. I have 9 220 Sharp ND series PV modules on each string. What DC isolator would you recommend? The inverter is an SB3800.

edexlab · Jan 13, 2011

read 2.1.12 Dti guide says Dc isolator must be adjacent to or integrated within the inverter
also BRB says isolators to be a switch disconnector 712.537.2.2.5 does an Ess switch meet this?
I always use seperate isolators as i don't think it does, you can't lock it off ,they deteriorate (arcing) every time they're pulled, i know this won't be frequent ,unless you are fault finding .

Markc · Jan 14, 2011

francie said:
Thank you for advice. I also feel better about fitting DC isolators as well. As I have 2 strings going into the inverter, I assume I use one DC isolator. I have 9 220 Sharp ND series PV modules on each string. What DC isolator would you recommend? The inverter is an SB3800.

One isolator per string so two isolators. As for size you need to work out total voltage and amperage of the DC per string and select a DC isolator that exceeds that.

JulianC · Jan 14, 2011

francie said:
As I have 2 strings going into the inverter, I assume I use one DC isolator.

You can use one isolator and feed both strings into it, providing the isolator can handle both the voltage (Voc x 1.15) and current (Isc x 1.25 x 2) demands. Nicer job to use two isolators though - one per string.

Have a look at Kraus & Naimer for DC isolators.

edit: D'oh, beaten to it.

Markc · Jan 14, 2011

JulianC said:
Have a look at Kraus & Naimer for DC isolators.

edit: D'oh, beaten to it.

Don't these come with pre fitted links to carry across all four contacts? (one on each side to make two pole) Can you remove these to make them 2 x two pole? If so I never knew you could do that.

JulianC · Jan 14, 2011

According to our K&N area rep you can remove/rejig the links to make the isolators two pole. However, he was telling me that people weren't confident in doing this so K&N were looking to release a 'pre-modified' multipole version of their isolators in the not too distant future. He wasn't sure exactly when that would be though.

Markc · Jan 14, 2011

JulianC said:
According to our K&N area rep you can remove/rejig the links to make the isolators two pole. However, he was telling me that people weren't confident in doing this so K&N were looking to release a 'pre-modified' multipole version of their isolators in the not too distant future. He wasn't sure exactly when that would be though.

Thanks Julian. I'm not adverse to using one isolator for two strings if we can. I even think it is better than two. Be interesting in seeing what comes out. We use K&N all the time, 1 for price and 2 for quality.

Noshocks · Jan 15, 2011

You cant remove the links as the contact gap for PV specified DC is not met, these isolators are AC/DC they have been modified specifically for PV in that the links added allow the minimum contact gap allowed for PV by using the outside contact.