Metal CU, TT earth and wiring for SPD

Won’t say it was normal, you should have a time delay RCD in place of the main switch to protect the tails. Also use DP RCBO’s for TT if your not using a dual RCD board. 

There is another reason you need a time delay rcd as the main switch on a TT system. As the rcd element of a 30mA rcbo will operate at some point between 15mA and 30mA, with a 10 rcbo consumer unit, that gives a worst case 300mA of leakage. Your earth rod impedance must therefore be better than 167 ohms. With more rcbo devices, your figure will be lower. 

Regards,

Alan. 

If a time-delayed (S type) RCD is used as the main switch, then it's ok to connect the SPD on the load side of the RCD (534.4.7)

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

How do you carry out safe isolation to work on a lighting circuit in a TT installation?

I come across many TT installations here in the sticks. I normally install a time delayed 100mA R.C.D. in an insulated enclosure before new all metal consumer units, and then R.C.B.O.s in the new consumer units. I have had success with this method in all cases but one.

I had nuisance tripping of the 100mA time delayed R.C.D. in a small domestic installation. All circuits tested good and no 30mA R.C.B.O. tripped off, just the pre consumer unit 100mA time delayed R.C.D.

I changed it for a D.P. main switch to avoid continual nuisance tripping. The reason for the tripping is unknown, but the home is near to Bacton where there are large radio masts at the gas terminals. A faulty R.C.D? I don't know. It tested out o.k. during testing.

Z.

Bedtime reading…..

RCD Nuisance Tripping: Who’s Guilty and What Needs to be Done? (globaljournals.org)

Z.

Zoomup: 
 

I come across many TT installations here in the sticks. I normally install a time delayed 100mA R.C.D. in an insulated enclosure before new all metal consumer units, and then R.C.B.O.s in the new consumer units. I have had success with this method in all cases but one.

Seems sensible to me, but you don't need an all RCBO board.

It may even be possible to combine it with SPD. https://www.tlc-direct.co.uk/Products/WYREC2SPD.html?source=adwords&ad_position=&ad_id=315107931576&placement=&kw=&network=u&matchtype=&ad_type=&product_id=WYREC2SPD&product_partition_id=987624279487&campaign=shopping&version=finalurl_v3&gclid=EAIaIQobChMIroCC5ojT8QIVyJ7tCh11ngDlEAQYASABEgKwDvD_BwE

In relation to Z's tripping RCD. I had a 100mA S type trip before one of the 30mA RCDs it supplied when the new HF fluorescent were turned on. Both RCDs passed all standard tests. One of the new HF fluorescents packed up and the 100mA RCD supplier sent me a replacement. Changed both at the same time and problem disappeared.

I certainly like Alon Capon`s point here