Metal CU, TT earth and wiring for SPD

I agree with Colin.

However I do have some reservations about TT and about TT & metalclad consumer units.

This is a personal thing and I am used to practically all systems I haved worked on being in TN land. I`m sure others will disagree with some of what I have to say.

And yes TN sometimes has its problems too.

Leaving aside the difference in types of RCD for a moment the two things that worry me about RCDs are the possible failure rate might be as high as 7%. It is accepted in some circles that life saving protection offered by correctly working 30mA RCDs might successfully save 95% of the population.

Marry the two figure together and the result could be around 12% failure to adequately protect.

Don`t get me wrong I think RCDs are marvelous things and worth their weight in gold.

I just do not like total reliance if that can be avoided sometimes it can`t be.

How do we mitigate the risk.

Well plastic consumer units gave us an edge there in some respects to some degree.

Cascading RCDs is another good way of reducing those percentages.

Purely in arithmetic land two cascaded RCDs at 7% risk might give us 0.49% risk of both failing.

In practice though "stiction" might occour to both for the same reasons so the risk might creep up from our 0.49% to nearer 7% .

Having them in slightly different locations and of different makes might mitigate this figure a little bit too.

We often do not cascade two 30mA RCDs for practical reasons, therefore we make one of them time delayed and decrease sensitivity by a facture of 3 or more. The usual combination being 100mA Delayed and 30mA non delayed (however we could use 30mA & 10mA or even 3mA & 10mA if our pockets are deep enough) . We tend to not view 100mA and above as not being good for shock risk purposes.

Then you need to weigh the benefits of SPDs against the types and failure modes and again TT could have more dangers if you don`t get the best type for TT usage.

I can see your concerns.

I suspect others will have more to say on this one.

Edit - two others beat me to it

I agree with Colin.

However I do have some reservations about TT and about TT & metalclad consumer units.

This is a personal thing and I am used to practically all systems I haved worked on being in TN land. I`m sure others will disagree with some of what I have to say.

And yes TN sometimes has its problems too.

Leaving aside the difference in types of RCD for a moment the two things that worry me about RCDs are the possible failure rate might be as high as 7%. It is accepted in some circles that life saving protection offered by correctly working 30mA RCDs might successfully save 95% of the population.

Marry the two figure together and the result could be around 12% failure to adequately protect.

Don`t get me wrong I think RCDs are marvelous things and worth their weight in gold.

I just do not like total reliance if that can be avoided sometimes it can`t be.

How do we mitigate the risk.

Well plastic consumer units gave us an edge there in some respects to some degree.

Cascading RCDs is another good way of reducing those percentages.

Purely in arithmetic land two cascaded RCDs at 7% risk might give us 0.49% risk of both failing.

In practice though "stiction" might occour to both for the same reasons so the risk might creep up from our 0.49% to nearer 7% .

Having them in slightly different locations and of different makes might mitigate this figure a little bit too.

We often do not cascade two 30mA RCDs for practical reasons, therefore we make one of them time delayed and decrease sensitivity by a facture of 3 or more. The usual combination being 100mA Delayed and 30mA non delayed (however we could use 30mA & 10mA or even 3mA & 10mA if our pockets are deep enough) . We tend to not view 100mA and above as not being good for shock risk purposes.

Then you need to weigh the benefits of SPDs against the types and failure modes and again TT could have more dangers if you don`t get the best type for TT usage.

I can see your concerns.

I suspect others will have more to say on this one.

Edit - two others beat me to it