Why I am sceptical of TT and RCDs

[Lectrician]

Why I am sceptical of TT and RCDs

Caravan park toilet block. Supply to block on a 6mm 2 core SWA, from a TT source, 40amp fuse.

Inside this plastic box outside behind toilet block is just an RCD, then 6mm T&E into boiler room, to a straight 4 way board. Crap design, and poor workmanship.

6mm T&E has rodent damage, causing a good short to earth. RCD has failed. (Outside in extremes of temperature, probably never tested and rarely operated).

Fault current would of been around 50 or 60 amp as Zdb was around 3 or 4 ohm. While you would hope the fuse would eventually blow, it didn't. It just burnt away the 2.5mm earth connection lugged to the banjo with a blue eyelet, causing metalwork of installation to come live. Site maintenance received a shock while filling water system for the first time this season after winter drain down.

TT and RCDs can be nasty.

 

[ricicle]

Has the site never been tested then Lec ?

 

[securespark]

Is that the ratty 6 milli - that stuff tie-wrapped to the unistrut?

 

[ericmark]

I fail to see this is anything to do with TT it would not have mattered what earth type.

Blue crimp = 2.5mm but minimum cable size for 40A is well over that size.

However I do see the point about RCD and temperature I had not considered that with electronic components there must be a temperature range.

 

[plugwash]

I fail to see this is anything to do with TT it would not have mattered what earth type.

With a TN system it is normal to engineer the system so that overcurrent protection devices provide the primary protection against short circuits with the RCDs acting as backup protection. With TT systems you are nearly always totally reliant on the RCD as you can't get the EFLI low enough to reliablly trip the overcurrent protection.

 

[JohnW2]

Why I am sceptical of TT and RCDs ... TT and RCDs can be nasty.

I'm a little confused here. What you describe is the consequence of a failed (and probably rarely, if ever, tested) RCD, not of the concept of protection which relies on an RCD and a TT earth.

If anything, your case might be taken as an argument for not relying on a single RCD. Your point about the environmental conditions affecting the RCD could be relevant, and one certainly should ensure that any device is fit to be used in the prevailing environment.

However, MCBs also theoretically could, and in practice sometimes almost certainly do, fail - but we don't really know because we can't test them.

Kind Regards, John

 

[Lectrician]

With the amount of testing I do, finding faulty RCD's is far from uncommon! This is why I posted.

The raggly bodged 6mm is the T+E cable tied to the post.

An RCD sat there faulty, and a L to E fault developing is a shock hazard plane and simple.

Eric - Of course it matters if its TT or not. If it was TN, the earth loop would be low enough to clear the fault - nice high fault current, fuse/MCB would operate. In a TT, it doesn't. A nice current just sits and flows.

This site is theoretically tested every year by a national un-named company.

 

[JohnW2]

With the amount of testing I do, finding faulty RCD's is far from uncommon! This is why I posted.

Fair enough. As I said, what you described was the consequence of a failed RCD, not of the concept of RCD/TT protection, per se.

We've had many discussions here about the reliability (or otherwise) and failure rates of RCDs and, despite differing views (and 'evidence') it seems clear that there is an appreciable in-service failure rate - so, as I said, theoretically perhaps a reason to question the wisdom of relying on a single RCD for fault protection. However, as I also said, I think we might be fooling ourselves into thinking that in-service 'failure' of MCBs is far more rare than it actually is - simply because we generally can't test them.

There are plenty of domestic installations which, for better or worse, are not provided with a TN earth. In such a situation, one has no choice but to rely on TT and RCDs, perhaps 'doubled up' if one is concerned about reliability.

Kind Regards, John

 

[plugwash]

Caravan park toilet block. Supply to block on a 6mm 2 core SWA, from a TT source, 40amp fuse.

If the supply is from a TT source shouldn't there have been a time delay RCD at source as well?..............

 

[Lectrician]

There is an MCCB at the origin with an earth VGI attached The settings on this have been teaked to their highest settings, and even then, suspect faulty.

Complete can of worms.

Its handed over to those responsible for the site, we only did an out of hours sunday response.