The Value of R.C.D.s

Zoomup: 
Are you always in control Chris? Sometimes the unexpected happens and then we need all the help that we can get. This is especially helpful if it is automatic and swift in operation.

The last time that driving put the wind up me was on the M6 by Lancaster in good Lancashire weather. Cruise control was on. Then there was a puddle and the car started going sideways (well, not straight ahead) and I had to turn off the cruise control before it drove me into a lorry. New car has active lane assist, so sometimes, if I cross the white line it yanks the car sideways. Most disconcerting. Imagine that in F1!

But I wanted to avoid motoring analogies because I don't think that they are appropriate.

Here at Kelly towers the lighting circuits and upstairs power including the electric shower are NOT RCD protected  i know they should be but I can't afford to get it done also the lighting in my disco room is protected by a plug in RCD which is better than nothing and yes I do test it regularly. My test bench in my shack workshop is protected by a 10 mA RCD and the whole shack us on a 30mA RCD.mincid3ntally the main DB despite its shortcomings was installed by  the SEB in about 2005

gkenyon: 
The requirements in BS 7671 are for additional protection to be provided (411.3.3, 411.3.4), not fault protection. In fact, additional protection is supposed to operate if fault protection fails (as well as basic protection)

That's exactly my point. In a sound installation, fault protection should not fail.

It's a bit like relying on air-bags instead of maintaining the brakes. Damn and blast - back to cars again! ?

I think we might have become a little confused here - No real reason to purposely omit RCD protection in a new install, but failing an otherwise sound but older installation?

Plus, we must not lose sight of the fact that RCDs are not the panacea which many believe them to be, they have their own shortcomings and limitations too.

 

Plus, we must not lose sight of the fact that RCDs are not the panacea which many believe them to be, they have their own shortcomings and limitations too.

Yeh but, yeh but. Yeh, but, Yeh…………but their upsides far outweigh their downsides if properly selected and installed.

Z.

We have an installation without RCD. How do we code it? It seems that some want C1, perhaps C2. Is this correct, lack of additional protection is potentially dangerous? Are you sure, or does this go with a “what if”?

Chris Pearson: 
That's exactly my point. In a sound installation, fault protection should not fail.

It's a bit like relying on air-bags instead of maintaining the brakes. Damn and blast - back to cars again! ?

My point is, it's more than simply failure of OCPD … it's about failure of other things like basic protection (things get broken, quite simply).

davezawadi (David Stone): 
 

We have an installation without RCD. How do we code it? It seems that some want C1, perhaps C2. Is this correct, lack of additional protection is potentially dangerous? Are you sure, or does this go with a “what if”?

I believe the current guidance from most providers of guidance (for TN-C-S and TN-S systems) is C2 or C3 (C2 in cases such as Bathroom, or where the socket-outlet is likely to be used for equipment outdoors, C3 otherwise). Can't be immediately dangerous as something else would have to be coded C1 already (failure of basic protection leading to access to exposed live conductors, for example). 

Example: https://www.electricalsafetyfirst.org.uk/media/2149/bpg4-1.pdf

Of course if it's TT, and loop impedances are too high for OCPDs to achieve disconnection times, this is also definitely a C2.

whjohnson: 
 

I think we might have become a little confused here - No real reason to purposely omit RCD protection in a new install, but failing an otherwise sound but older installation?

Plus, we must not lose sight of the fact that RCDs are not the panacea which many believe them to be, they have their own shortcomings and limitations too.

Agreed, this does not lead to C1, see previous post … but should be considered C2 or C3 according to most guidance out there.

By taste I like everything in a home, from soon after the meter, to have ≤ 30 mA RCD protection.  Apart from giving some backup for direct contact or broken earthing, and sometimes faster disconnection than OCPDs, it avoids the potential for lethal currents to leak to building structures, etc - there's the well known case in the UK, and a more extreme example here (Australia, double death, showers, concrete floor, nail: link).  For my own systems I just feel it unnecessary and unacceptable to avoid this simple addition. 

Doubtless some lives have been saved by RCDs, though probably a good deal fewer than we might think from looking at the correlation of higher RCD coverage and lower death rate over time.  I suspect that product standards have played a big role, e.g. avoiding grippable conductive parts.  Plenty of shocks happen unreported that don't cause death even without RCDs, so the cases of “I got a shock and it tripped - the RCD saved me” wouldn't necessarily have been fatal.  I wouldn't be surprised if there's been as much positive effect from RCDs on fire deaths as on shock deaths, particularly in systems with earths around or between live conductors; but those statistics are even more shaky. 

In Sweden RCDs were uncommon up to the 1990s or so, and I see many houses without them, often with old installations, outdoors extension leads, cars etc. (In contrast to the recent worries about car chargers in the UK, it's been thoroughly normal for decades to have cars's chassis earthed by their class-I engine-block heaters, and plenty of cars still do so in old installations with no RCD; besides this, they practically all lead back to a ‘PEN’, often within the customer installation in the older ones before the harmonized rules around the 1990s put a big minimum size on such conductors). In spite of all this, there have been years with no electric shock deaths at all (nationally, any cause) and the few that usually happen in a year tend currently to be electricians or line workers at work, or youths clambering on parked electric trains unaware of the 15 kV 16.7 Hz hazard in the wires on and above the train.  So although I sometimes subtly stick RCD plugs onto extension leads that I see branching to dozens of outdoors power tools, cement mixer, etc, I have to admit the statistics don't really justify a great need compared to other risks we accept.  My own tastes are based on its being such a simple addition “just in case".