DP Isolation.

[securespark]

This thread has been prompted by my reply to another thread.

OOI, How many of you are aware that when working on a TT-fed installation, it is imperative to use DP isolation.

Isolating the line conductor only is not sufficient to avoid the risk of electrical contact. This is due to the fact that a rod-fed earth cannot be relied upon to be at the same potential as the neutral.

 

[ricicle]

If using RCBOs on a TT aren't they supposed to be DP ?

 

[Click-Sure]

They are indeed supposed to be DP.

There seems to be quite a few people who neglect this, and other things besides, Only yesterday I saw a metalclad CU in a TT environment

PME conversion is pretty much the only way I advise people to go these days, assuming it is available, in most of my patch, it is.

And yes, the old 'what is my supply type' query works well, resulting in a visit from an engineer who invariably ends up changing the cutout

 

[securespark]

RCBO's don't comply, but not because of the SP argument.

The maximum disconnection time for TT systems is 0.2s.

The max. disconnection time for RCBO's is 0.3s

 

[Adam_151]

Not sure I buy that argument Simon...

If the Zs is low enough that the fault current is at least 5x then the disconnection time is 0.04

Anyway if you are saying RCBOs a no no because of disconnection times, then what are you using instead, AFAIK there is no difference in disconnection times allowed between RCBOS to BSEN61009 and RCDs to BSEN61008...unless you have a stock of old devices to BS4913?

 

[Spark123]

The fault current at which you test the device needs to be significantly higher than the trip rating of the device, originally the 17th edn regs had it down as 2 x I but dropped that with the corrigendum (probably due to a lot of irate test equipment manufacturers). So if you have a 30mA device and test it at say 100mA it needs to trip in less than 200ms, shouldn't be a problem really given it must trip within 40ms at 5 x I.

 

[securespark]

Not sure I buy that argument Simon...

If the Zs is low enough that the fault current is at least 5x then the disconnection time is 0.04

But would it be low enough on TT?

Anyway if you are saying RCBOs a no no because of disconnection times, then what are you using instead, AFAIK there is no difference in disconnection times allowed between RCBOS to BSEN61009 and RCDs to BSEN61008...unless you have a stock of old devices to BS4913?

Yup, fair point. I was forgetting for a brief second that 61008's have the same max trip time as the 4293's.

So why do the regs specify 0.2s for TT? It's a mystery.

 

[securespark]

The fault current at which you test the device needs to be significantly higher than the trip rating of the device, originally the 17th edn regs had it down as 2 x I but dropped that with the corrigendum (probably due to a lot of irate test equipment manufacturers). So if you have a 30mA device and test it at say 100mA it needs to trip in less than 200ms, shouldn't be a problem really given it must trip within 40ms at 5 x I.

Sorry 123, I missed your post off the bottom.

So you're saying that if you test a 30mA 61008 or 61009 device @ 100mA and it trips within 200ms, then your installation complies with the 0.2s reg?

 

[Spark123]

You only need 150mA flowing to have a 40mS disconnection of a normal 30mA RCD - this equates to something like having a Ze of 1533 ohms!
Of course a TT shouldn't have a Ze above 200 ohms for reliability.
61008 and 61009 have a 300mS maximum disconnection time at I/\n, BS4293's have a max disconnection time of 200mS at I/\n.
They must all trip within 40mS at 5xI/\n for a standard device (30mA max).

 

[Spark123]

The fault current at which you test the device needs to be significantly higher than the trip rating of the device, originally the 17th edn regs had it down as 2 x I but dropped that with the corrigendum (probably due to a lot of irate test equipment manufacturers). So if you have a 30mA device and test it at say 100mA it needs to trip in less than 200ms, shouldn't be a problem really given it must trip within 40ms at 5 x I.

Sorry 123, I missed your post off the bottom.

So you're saying that if you test a 30mA 61008 or 61009 device @ 100mA and it trips within 200ms, then your installation complies with the 0.2s reg?

Yep

 

[securespark]

Fair enough. Sorted. Cheers!