Supply Characteristics Advise needed

[Bendy spring]

Try disconnecting earth from TNS measure ZE with only earth stake if its still high try relocating earth stake if still high add another stake so you have 2 to bring the ZE down, 59 ohms is way too high for a TNS and not safe, and 500 ohms is way too high for a TT.

 

[WayneL]

Not if you have a 100mA selective RCD on the front end and 30mA RCBO's on the rest....

30mA RCBOs are for additional protection against electric shock and therefore wouldn't necessarily feature on every circuit (e.g. dedicated socket for fridge/freezer...smokes).

If an earth fault occured on one of these circuits, it would take out the front end, 100mA RCD and, therefore the whole installation.
So, like I said, most TT system go against the 'inconvenience' regulation.

Also, you can't state that the 501ohm Ze is 'completely against the regs'
The regs state that the resistance of the earth electrode should be as low as practicable and that a value exceeding 200 ohms may not be stable.

It doesn't say you can't have a higher Ze than this, it just means that the impedance readings may fluctuate if higher than 200 ohms and contravene 542.2.2

As long as you meet the Zs values in table 41.5 for your protective device, then you aren't breaking any regulations

 

[s1m0n]

Hi
as it stands now, treating installation as a TT
with additional earth rod Ze is now 59.2 ohms

the consumer unit is BS3036 rewireable fuses

the 80A 30mA 2 pole RCD is protecting the whole istallation, (Note these were in place aready)

So that should satisfy the regs,

Discrimination can only be achieved by upgrading the Consumer Unit, which I have recommended.

 

[pevvers]

30mA RCBOs are for additional protection against electric shock and therefore wouldn't necessarily feature on every circuit (e.g. dedicated socket for fridge/freezer...smokes).

If an earth fault occured on one of these circuits, it would take out the front end, 100mA RCD and, therefore the whole installation.
So, like I said, most TT system go against the 'inconvenience' regulation.

Also, you can't state that the 501ohm Ze is 'completely against the regs'
The regs state that the resistance of the earth electrode should be as low as practicable and that a value exceeding 200 ohms may not be stable.

It doesn't say you can't have a higher Ze than this, it just means that the impedance readings may fluctuate if higher than 200 ohms and contravene 542.2.2

As long as you meet the Zs values in table 41.5 for your protective device, then you aren't breaking any regulations

All good points mate, and me saying that a Ze of 501 ohms is completely against the regs was probably the wrong way to put it. What I should have said is that it is not recommended, and I don't know of a sparks who would be happy with that and just take it like it is.

Minimising the inconvenience in the event of a fault is just that, minimising. In a TT system, yes, you do have your hands tied somewhat, which is why it's minimising, and not eradicating totally. But surely by having a 100mA S type up front, plus 2 x 30mA, this would be minimising as you would be a lot better off than if you just stuck with the 30mA that's currently up front....

Anyone seeing this on a PIR would only code it as a 4, but I would still recommend it's changed, as one piece of slightly faulty equipment and then it's lightss out for the occupants.

The only way to totally conform with the 17th reg of minimising would be to install complete 30mA RCBO protection on a TT, as long as the CU is insulated, but I'm sure you know as well as I do that not many people will pay the extortionate prices for all RCBO's, which is why the split is the done thing.

Just my thoughts, but thank you for the differing views as I like a good discussion!!

 

[WayneL]

All good points mate, and me saying that a Ze of 501 ohms is completely against the regs was probably the wrong way to put it. What I should have said is that it is not recommended, and I don't know of a sparks who would be happy with that and just take it like it is.

Minimising the inconvenience in the event of a fault is just that, minimising. In a TT system, yes, you do have your hands tied somewhat, which is why it's minimising, and not eradicating totally. But surely by having a 100mA S type up front, plus 2 x 30mA, this would be minimising as you would be a lot better off than if you just stuck with the 30mA that's currently up front....

Anyone seeing this on a PIR would only code it as a 4, but I would still recommend it's changed, as one piece of slightly faulty equipment and then it's lightss out for the occupants.

The only way to totally conform with the 17th reg of minimising would be to install complete 30mA RCBO protection on a TT, as long as the CU is insulated, but I'm sure you know as well as I do that not many people will pay the extortionate prices for all RCBO's, which is why the split is the done thing.

Just my thoughts, but thank you for the differing views as I like a good discussion!!

I totally agree, mate. I think the big change will come when they manage to reduce the price of RCBOs. This is definitely the way things will go in the future - individual circuit protection - there are too many problems the way things are done now.

 

[pevvers]

It doesn't say you can't have a higher Ze than this, it just means that the impedance readings may fluctuate if higher than 200 ohms and contravene 542.2.2

Also, done a PIR on a property once, Ze of around 800 ohms. 30mA RCD did not trip, on test button or test instrument. Saying that it can have a maximum of 1667 ohms and still comply with the regs is fine, but the RCD won't work from my experience. I'm not sure of the science behind it, but I'll probably have a look into it.

Banged in a new earth rod and the RCD worked fine so it wasn't a faulty device....