RCD Trips

Too many loaded questions/comments Glen, in my opinion....I would hazard a guess that you know what the guy is getting at though to be honest. The fact is that TL inverters have an inherent AC leakage current, ask each manufacturer what it is and they will normally tell you. Let's say, for argument's sake, that it is 20mA. On start up, especially on moist/misty mornings, where parasitic capacitive leakage currents can be naturally generated by the PV array, these currents when combined with the inherent leakage current generated by the TL inverter can exceed the 30mA trip limit and "nuisance" trip the RCD. SMA and others recommend a trip limit of at least 100mA because of this. PowerOne recommend 300mA.

Is the discussion about whether it is possible to increase the RCD trip limit without contravening any applicable requirements for "human protection" under BS7671? Or is it about whether TL inverters have inherent leakage current? No-one suggested that the auto-response was the increase the trip limit regardless. I think there was a mention of conforming to reg's and/or having a TT supply.....

I'm pleased to read that you use Type-B RCDs when fitting TL inverters, can I ask whether you would like to share why you do that please? Is that something you do for all TL systems or just ones where you deem it necessary to fit an RCD according to BS7671? Just curious, thanks.

A 30mA RCD can trip at anything above 15mA. Standard tests ensure that it doesn't trip at 15ma but does at 30mA and 150mA, so even the 20mA startup leakage could be enough to trip!

My response was really got serious with this comment

any tl inverter will take out 30ma rds all day long. Either scrap the rcd all together of change to 100ma at least - obviously provided installation method of cable allows or its a tt system

Click to expand...

Scrap the RCD all together, OK lets do that,

On the serious note why do we use Type B RCD, if you do or if you dont this is up to the installer, there are so many shades of grey in regs, so if I install a sunnyboy and the protection fails the type B RCD covers me under governing law. we can argue manufacturers guidelines supercede all other regulations but I can't really be bothered wasting a day in court to explain that. I can't really be bothered explaining to trading standards how it all works and reading manuals to them.

If it does not hinder the performance of the system then no harm done!!

My sb4000tl has a 30ma rcd

It has not tripped as yet since November. Is it likely to?

yellowvanman said:

A 30mA RCD can trip at anything above 15mA. Standard tests ensure that it doesn't trip at 15ma but does at 30mA and 150mA, so even the 20mA startup leakage could be enough to trip!

Click to expand...

Indeed!

Glen Powell said:

My response was really got serious with this comment



Scrap the RCD all together, OK lets do that,

On the serious note why do we use Type B RCD, if you do or if you dont this is up to the installer, there are so many shades of grey in regs, so if I install a sunnyboy and the protection fails the type B RCD covers me under governing law. we can argue manufacturers guidelines supercede all other regulations but I can't really be bothered wasting a day in court to explain that. I can't really be bothered explaining to trading standards how it all works and reading manuals to them.

If it does not hinder the performance of the system then no harm done!!

Click to expand...

Completely agree! Most on here would know that I am a proponent for the use of the DC sensitive RCDs where TL inverters are concerned.....good to get someone else's take on it.

wstevew said:

My sb4000tl has a 30ma rcd

It has not tripped as yet since November. Is it likely to?

Click to expand...

Depends what type it is.....

Not sure it just says. RCCB 30ma. It's in it's own mini CU.


From memory sunny explorer usually displays residual current of 8ma never seen it change.

Forgot to post the new RCD decision process as recommended in the new Guidance Note 7:

That decision tree matches with how I see it.

A type B RCD is only required for fault protection if the inverter hasn't got means of preventing leakage of DC current onto the AC side.

Bit too simplistic if you ask me.....not entirely sure that the statement "isn't able to feed DC fault currents into the AC side of the installation" is all-encompassing or enough to satisfy all potential scenario's where DC fault currents might exist. I'll take the fifth on this for now I think....