Was this the right decision?

[moggy1968]

reg 411.5

 

[VMan]

I would recommend talking to Sibberts. I would say fitting a 100ma time delayed type B RCD might do the trick but your going to have to cut the meter tails and put them into a henley block in order to seperate it from the household electrics.
Good choice for your first one!! Welcome to the vertical learning curve society:banghead:

I dont believe it needs to be type b as its an isolating transformer not transform-less inverter

 

[sambotc]

Vertical learning curve sounds about right. Thrown in at the deep in is another way of putting it. In my defence I do at least try and get things right and safe even if it doesn't always work out that way!

So I take it the issue is the fact that a fault current of 100ma or more would effectively knock out both RCDs simultaneously or that any fault current from the PV could stop the 100ma RCD from tripping under fault conditions? Just trying to picture the chain of events but if i'm honest I cant figure it out in this loaf of mine.

 

[moggy1968]

I dont believe it needs to be type b as its an isolating transformer not transform-less inverter

Thats why I suggested talking to Sibberts as this is a bit beyond my level. I would think a type B but am quite happy to be proved wrong!
It would certainly keep the cost down!

 

[moggy1968]

Vertical learning curve sounds about right. Thrown in at the deep in is another way of putting it. In my defence I do at least try and get things right and safe even if it doesn't always work out that way!

So I take it the issue is the fact that a fault current of 100ma or more would effectively knock out both RCDs simultaneously or that any fault current from the PV could stop the 100ma RCD from tripping under fault conditions? Just trying to picture the chain of events but if i'm honest I cant figure it out in this loaf of mine.

basically yes, and well done for asking, most people would probably just stick their head in the sand and not give a ----!!

 

[sambotc]

Thanks Moggy, is the 2.5mm CSA main earth OK then? If you notice I quoted a couple of lines out of the OSG in a previous post, could you just verify that I am wrong, or if I have misunderstood that sentence from the OSG?

To be honest, having to do what I did was a bit of a chore for the end result, and given the fact it was below 200ohms anyway, but still to high to get any results on disconnection times it did seem like a pointless exercise, but reg's are reg's after all!

Cheers, Sam

 

[screwdriver]

depends what book you read but general recommendation is less than 100 ohms for tt system (some recommend 200 ) but in effect you are talking about bringing this down to allow an mcb to trip under earth fault conditions, this is almost impossible to achieve with TT systems unless you are next to the sub station.
any earth fault is dealt with by the rcd, this generally makes the zs readings a bit meaningless if you are trying to compare to max zs for the circuit. what you sometimes find is that the zs is so high that there is less prospective fault current available to trip the mcb than the mcb rating but it still complies!

Looking at the job you had to do I would have been tempted to fit the supply from the rcd switch into a henly which fed a 30ma rcd switch feeding the household circuits and then connected the inverter to a little cu also connected to the henly.

That way you are increasing the safety of the household circuits (albeit at risk of increasing nuisance tripping) and also avoiding using a 30ma for the inverter.

 

[screwdriver]

Ok thanks for the replies, I see what you are saying now and it makes sense regarding a type B RCD. As for earth size, OSG (green) table 4.4ii, beneath this table it states

Notes:
1 assuming protected against mechanical corrosion by a sheath

2 The main protective bonding conductors shall have a cross sectional area at least of not less than half that required for the earthing conductor and not less than 6mm

Am I interpratating that wrong then?


Thanks in advnce

It refers to protective bonding conductors not being less than 6mm (the ones going to the gas and water)

the earthing conductor can be quite small as long as its protected from corrosion and damage. It will only ever need to carry 100ma so it isn't really working too hard!

 

[yellowvanman]

Thanks Moggy, is the 2.5mm CSA main earth OK then? If you notice I quoted a couple of lines out of the OSG in a previous post, could you just verify that I am wrong, or if I have misunderstood that sentence from the OSG?

Actually if the 2.5mm earth is buried it should have mechanical protection and protection against corrosion (sheath), if not should b larger (Table 54.1)

As to size of earth conductor, there are 2 methods -

1) Using Table 54.7. So if your tails were 16mm, earth conductor needs to be 16mm.

2) Other method is to use adiabatic equation (543.1.3), which is the more effecient in terms of copper useage. With a high Ze so fault current (I) will be restricted to a low value and hence CSA of earth will be very small, but your limited to 2.5 as a minimum.

 

[sambotc]

So why the need for 6mm min for bonding, but 2.5mm for main earth, surely that doesn't make sense or am I missing something here?

Well had my first experience in fitting an earth spike at least, oh the joys of bouncing off the footings for the first 3 or 4 go's!