Cable from loft to CU

[moggy1968]

huh?.....

 

[SibertSolar]

Nuisance tripping tends to result from a combination of parasitic capacitive leakage from a floating DC array with the inherent AC leakage current from a TL inverter. If you must fit a 30mA RCD then using a TX inverter will avoid nuisance tripping.....

 

[Worcester]

Nuisance tripping tends to result from a combination of parasitic capacitive leakage from a floating DC array with the inherent AC leakage current from a TL inverter. If you must fit a 30mA RCD then using a TX inverter will avoid nuisance tripping.....

Reminds me of the trailer for NCIS

 

[Gavin A]

Nuisance tripping tends to result from a combination of parasitic capacitive leakage from a floating DC array with the inherent AC leakage current from a TL inverter. If you must fit a 30mA RCD then using a TX inverter will avoid nuisance tripping.....

true, though as I say it's not been an issue once so far, and I don't expect it to be using SMA or Power-One - not so sure about the cheaper chinese models.

I'd never install them to a shared 30mA RCD though, asking for call backs that is.

 

[SibertSolar]

No, you're right Gavin, never on a shared RCD. Most TL manufacturers will tell you that their unit exhibits about 20mA AC leakage under normal operating conditions. Odd though, that SMA "recommend" a 100mA min trip limit (to avoid nuisance tripping) and I could be mistaken but didn't P1 always recommend a 300mA trip limit, per inverter - although that could be due to the fact that most of mainland Europe (especially Germany curiously) don't consider a 100mA trip limit RCD as a standard rating, normally using either 30mA or 300mA.

So, we know in practice that a 30mA limit on a dedicated circuit is generally ok with a good quality TL unit (damp mornings are when a problem will typically materialise as we know) but on the other hand we have manufacturer's recommendations to follow....what to do? ;-)

 

[sedgy34]

Put in a 6mm inverter manufacturers advise 6mm t&e so the resistance is under 1ohm

 

[Gavin A]

No, you're right Gavin, never on a shared RCD. Most TL manufacturers will tell you that their unit exhibits about 20mA AC leakage under normal operating conditions. Odd though, that SMA "recommend" a 100mA min trip limit (to avoid nuisance tripping) and I could be mistaken but didn't P1 always recommend a 300mA trip limit, per inverter - although that could be due to the fact that most of mainland Europe (especially Germany curiously) don't consider a 100mA trip limit RCD as a standard rating, normally using either 30mA or 300mA.

So, we know in practice that a 30mA limit on a dedicated circuit is generally ok with a good quality TL unit (damp mornings are when a problem will typically materialise as we know) but on the other hand we have manufacturer's recommendations to follow....what to do? ;-)

ask the manufacturer - power-one confirmed they don't expect to see any problems from 30mA when I challenged them on it at an installer training session, and don't have a problem with it being used as long as it's a single inverter, which it virtually always will be with a dual MPPT inverter.

Obviously some 30mA RCDs can in theroy be as low as 15mA trips, which does get uncomfortably close to the peak earth leakage from those inverters, which I guess is why they recommend 100mA, but we must be talking in the region of 1% or lower that might have issues when earth leakage is at its highest, and it's a simple enough job to swap it out if it ever does become an issue on an install.

Having seen the event logs from multiple SMA 4000TL / 3600Tl inverters, I'd expect that the all pole RCMU function in the inverter actually prevents this becoming an issue for the 30mA RCD we're fitting, as it's certainly logging that it's doing something on those damp mornings and evenings.

 

[Gavin A]

tbf, we have used 100mA on installs that are the other end of the country from us, as even a 1% risk of having to do that journey to swap out the 30mA RCD really isn't worth it.

 

[Octopus]

Ok. So 6mm is the way to go...

My next question is using SWA a good idea so that the cable run doesn't need RCD protection?

 

[SibertSolar]

Haha, you trolling Murdoch??.... ;-)

 

[sedgy34]

I would put 6 mm t&e in and cap it with 3 mm sheathing

 

[SolarCity]

Ok. So 6mm is the way to go...

My next question is using SWA a good idea so that the cable run doesn't need RCD protection?

Depends where you're running it. Sometimes it is best to run the cable on the outside of the house (although no need to do this with a rewire - which is what I assume you are doing) If you can get the cable into the loft (or garage) without sinking the cable in then you shouldn't need SWA.

 

[Octopus]

Depends where you're running it. Sometimes it is best to run the cable on the outside of the house (although no need to do this with a rewire - which is what I assume you are doing) If you can get the cable into the loft (or garage) without sinking the cable in then you shouldn't need SWA.

The cable would be added as part of a major refit, so yes its a rewire, and no I don't want a cable on the outside of the house unless I have to, or trunking on the inside.

 

[sedgy34]

Do post 41 then

 

[Octopus]

Do post 41 then

Yup, sounds like a plan!