Earth leakage circuit breaker

[goasis]

From a brief bit of Internet research my understanding is earth leakage circuit breakers monitor voltage on the CPC rather than an inbalance in current between L &N as an RCD does. Is this correct?

 

[brianmoooore]

That is correct. The problem with them is that parallel earth paths tend to bypass them and make them less effective.
My policy for at least thirty years has been to replace on sight.

 

[timhoward]

While we are quizzing older and wiser people....
Was there a link between ceasing to be able to rely on water pipes as earth electrodes for TT systems and the introduction of VOELCBs?
I was quizzed the other day by a retired physics teacher on the history of TT systems before up-front RCDs and wasn't totally sure of which parts related to which other parts.

 

[pc1966]

The VOELCB pre-dates plastic pipes by a long time!

For rural areas you might not have water mains supply, etc, anyway, it might be brought in by buckets, etc (I kid you not). Even if you did, it is not always going to be low enough to blow a 30A rewirable fuse for a cooker circuit!

 

[pc1966]

Strictly speaking the VOELCB is not voltage-operated, it is the current through the trip coil that fires it, but as that coil was in the hundred ohm region, it needed several volts to operate it.

 

[timhoward]

The VOELCB pre-dates plastic pipes by a long time!

For rural areas you might not have water mains supply, etc, anyway, it might be brought in by buckets, etc (I kid you not). Even if you did, it is not always going to be low enough to blow a 30A rewirable fuse for a cooker circuit!

Thanks!
So was there ever an era when no up front device at all protected TT installations?
Or maybe (guessing) an era where earth electrode resistance had to be significantly lower that it needs to be today?

 

[pc1966]

Thanks!
So was there ever an era when no up front device at all protected TT installations?
Or maybe (guessing) an era where earth electrode resistance had to be significantly lower that it needs to be today?

I am sure there was, but probably before the 1940s or so.

It is very hard to get a low enough earth impedance to blow even a 5A fuse, so by time the need for ADS was established then I think the VOELCB became the norm in rural areas where a 3rd conductor for E was impractical for cost or physical reasons.

@Lucien Nunes might know such details.

 

[M-Ty]

A quick reminder that RCCBs used to be known as current-operated ELCBs.

 

[brianmoooore]

And RCDs used to be known as RCCBs

 

[Mike Johnson]

The VOELCB pre-dates plastic pipes by a long time!

For rural areas you might not have water mains supply, etc, anyway, it might be brought in by buckets, etc (I kid you not). Even if you did, it is not always going to be low enough to blow a 30A rewirable fuse for a cooker circuit!

You may be surprised or maybe not, a lot of French properties are still using well water, but now days pumped up to a header tank, no bucket required.

 

[pc1966]

And RCDs used to be known as RCCBs

They still are in many places!

Though many use RCCB to mean just the residual current protection as a stand-along device, RCBO when it has over-current as well, and RCD to mean the residual part in either case.

 

[LastManOnline]

While we are quizzing older

Right, I qualify for this bit

and wiser people....

Not sure about this bit!

Was there a link between ceasing to be able to rely on water pipes as earth electrodes for TT systems and the introduction of VOELCBs?
I was quizzed the other day by a retired physics teacher on the history of TT systems before up-front RCDs and wasn't totally sure of which parts related to which other parts.

Correct. A TT system without sufficiently low RA to disconnect an OCPD requires an earth leakage device in order to function.
Did, nt get too many lessons on the history of earthing when I was serving my apprenticeship but my impression is that earthing as a concept only really got going from the 1950,s onwards. Up until that point electrical safety relied heavily on a "Don, t touch it" approach.
The only TT system I am aware of that has a sufficiently low RA to disconnect an OCPD (without using metallic service pipes) is to be found in mainland Europe

 

[brianmoooore]

I reckon a 1950s property with main earth wire connected to half a mile or so of buried copper water supply pipe, as many around here were, would have had a pretty low RA.

 

[pc1966]

One issue with TT earthing in general is how dependant it is on the soil resistivity, and that is often down to moisture levels and so how deep you can go. To get, say, 0.4s disconnection reliably on the old 30A rewirable fuses you need something like 0.7 ohms for the rod (including whatever Earth impedance there is at the supply end).

Short of a buried steel foundation that is just not happening!

This article has some useful info and some handy plots. E.g. the buried horizontal conductor in 100 Ohm.m soil is not reaching 0.7 ohm even at 1km length!

Measurements and Calculations Of Earth Electrode Systems (BS 7430) | EEP

System earthing involves the provision of a connection to the general mass of earth. This connection should have a resistance not greater than that required to operate safety mechanisms to isolate the electricity supply from a fault situation.

electrical-engineering-portal.com

 

[LastManOnline]

I reckon a 1950s property with main earth wire connected to half a mile or so of buried copper water supply pipe, as many around here were, would have had a pretty low RA.

Yes. That is correct. That's why I mentioned in brackets(without use of metallic services)

 

[LastManOnline]

This article has some useful info and some handy plots. E.g. the buried horizontal conductor in 100 Ohm.m soil is not reaching 0.7 ohm even at 1km length!

Measurements and Calculations Of Earth Electrode Systems (BS 7430) | EEP

System earthing involves the provision of a connection to the general mass of earth. This connection should have a resistance not greater than that required to operate safety mechanisms to isolate the electricity supply from a fault situation.

electrical-engineering-portal.com

In practice the only countries that tend to use a low RA TT system are those with suitable soil conditions like the Netherlands. They can literally sink a copper wire vertically in to the ground without...
1)any obstructions in the way
2) permanently wet soil (sand)

 

[LastManOnline]

I reckon a 1950s property with main earth wire connected to half a mile or so of buried copper water supply pipe, as many around here were, would have had a pretty low RA.

Sorry. Misunderstood your reply.

 

[timhoward]

Thanks all for humouring my curiosity!

 

[Lucien Nunes]

Regarding the history...

Not all installations were always earthed. Although earthing and the TN supply concept was well established in principle by the 1920s there was not always a need for it. Many installations were either lighting only or for minimal use of appliances, many of which were class 0 and fitted with 2-pin plugs, so (especially with wooden casing and TRS installations) there was nothing that needed earthing.

All buried services were once metallic and of reliably low resistance, therefore any urban building with water laid on could get an amply low resistance earth. It was considered acceptable to use the water pipe as the main earth and no rod would be installed where a water pipe was available. In most pre-war domestic installations the largest circuit was 15A and indeed many had only 5A circuits on a 15A main supply or similar. Therefore no difficulty was encountered in clearing the fuse with an earth fault. Touch voltage and disconnection times were not the hot topics they are today; what mattered was that the fuse would blow.

Premises that did not have any earthed metallic services were significantly less likely to have a public electricity supply. Where present, there was the option of an installation that did not need earthing and/or had only low-rated circuits (as above, lighting points and 2-pin 5A sockets). Where electricity was generated on site, obviously the installation could be TN with the rod only needed as a potential reference. Electricity then being rarely used outdoors, a fault to real earth rather than CPC was less likely.

Obviously there were situations that needed a low Ra and didn't have one, but the impression I am trying to give was that these were not particularly common at first. Soon after the war, when increasing numbers of homes had a TN supply and more appliances, there was an increasing risk of the earthing being inadequate or missing. However, the ELCB was soon available as an option, so a properly executed installation was unlikely to be stuck without any acceptable solution. Improperly executed installations... well we still have those.