Was this the right decision?

[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!

 

[moggy1968]

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.

you still need to RCD your PV circuit, also using a single 30ma RCD for all circuits doesn't meet the requirement for discrimination so you would need to fit a 17th ed type board with divided circuits.
I recently had to relocate the board on a TT system (no PV) so had to protect the 5m of tails with a 100ma time delay RCD. The other time I have used this set up was on my folks house. The main board is in the loft, so I fed the garage supply with a 100ma time delay RCD then fitted a 30ma RCD on the garage board so hopefully if the garage tripped you didn't have to go up into the loft to reset the main RCD.
If your board is near the point of entry of course this isn't necessary.

 

[GaryM]

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!

It will quite easily carry more than 100mA under earth fault conditions.
An RCD is not a current limiting device. The higher impedance of the earth path results in a lower current flow hence TT systems generally get away with smaller earthing conductors.

 

[yellowvanman]

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?

No it doesn't make sense but maybe to others who know more than me it does!

 

[moggy1968]

it doesn't make sense, but thats the regs for you!

 

[screwdriver]

It will quite easily carry more than 100mA under earth fault conditions.
An RCD is not a current limiting device. The higher impedance of the earth path results in a lower current flow hence TT systems generally get away with smaller earthing conductors.

Sorry maybe I was tired an made an oversimplification , it won't carry more than 100ma for more than the time the trip is able to trip in! The impedance is another valid point.

 

[GaryM]

It will carry the whole earth fault current but only for the time the RCD takes to trip if the fault current is more than the rating of the RCD, 100mA in this case.

If there's a earth fault at the end of a circuit with a ZS of 35ohms then earth fault current will be 230/35 = 6.57A. As this is more than 100mA then the RCD will trip.