V O E L C B and C O E L C B
- Thread starter robbo1970
- Start date
- Joined
- 11 Jan 2004
- Messages
- 46,050
- Reaction score
- 3,581
- Country
That a shock could still be suffered and the unit would not trip.
Nothing wrong with a Current Operated ELCB, they are what an RCD was called before they were known as RCDs.
Voltage operated ELCBs relied on the installation having one reference to earth via the device to an earth rod, any parallel paths to earth such as the water / gas mains (if they are metal) via protective bonding can render a VOELCB useless.
Voltage operated ELCBs relied on the installation having one reference to earth via the device to an earth rod, any parallel paths to earth such as the water / gas mains (if they are metal) via protective bonding can render a VOELCB useless.
P
Paul_C
That's no different from the modern RCD, which is a current-operated ELCB by a different name. All that's happened is that it's undergone a couple of "official" name changes over the years, and the types installed today for general protection are more sensitive.
The voltage-operated ELCB seems to be much maligned these days, often unfairly I feel. To work as intended, the reference earth electrode had to be maintained completely outside the influence of any other earth electrodes, intentional or unintentional by way of bonded pipework etc. This was the main problem with the device in practice, when that integrity could not be maintained. Some debate about it here:
http://www.diynot.com/forums/viewtopic.php?t=264171&start=60
P
Paul_C
To be clear, that's only where there were overlapping resistance gradient areas between those bonded items and the reference electrode (or the cable running to it, hence why it was required for that cable to be insulated). You could have as many parallel paths to earth as you liked on the frame (installation) side of the coil, but so long as everything connected to the electrode side was completely outside the influence of those items, the ELCB would still do its job.
how did they expect it to work with immersion heaters though ?
These are earthed, and the tank can have an earth path back to the mains cold pipe.
These are earthed, and the tank can have an earth path back to the mains cold pipe.
It would do it's job in the sense of operating if the pd between the installation's CPCs and earth electrode was 50V (or whatever) or more. However, if (as in my installation) the resistance of the path to earth via parallel paths is much lower than via the earth electrode, then it's surely very possible for a lethal current to flow through the victim to earth (via parallel paths which are partially bypassing the coil) without the voltage across the ELCB coil rising to anythong like 50V?
Kind Regards, John.
The key point with VOELCBs was that as soon as the MET voltage raised above a set voltage above true earth they tripped, possibly 50v??
Even if there were parallel paths to true earth the voltage on the MET could still rise, more current would be required to do this to overcome the resistance of the parallel path.
As I think Paul C says parallel path via ground causing the earth rods voltage to rise would mean the NET voltage across the ELCB coil is reduced hence a larger voltage would be required to raise the NET voltage causing it to trip.
Even if there were parallel paths to true earth the voltage on the MET could still rise, more current would be required to do this to overcome the resistance of the parallel path.
As I think Paul C says parallel path via ground causing the earth rods voltage to rise would mean the NET voltage across the ELCB coil is reduced hence a larger voltage would be required to raise the NET voltage causing it to trip.
P
Paul_C
They were all connected to the "F" terminal of the ELCB, and thus to one end of the operating coil. The "E" terminal at the other end of the coil was connected only by way of an insulated conductor to the reference electrode. That's why it was important for that electrode to be kept well outside the area of influence from any of the pipework etc. which was connected directly to the frame terminal.
It was also important that the reference electrode be outside the area of possible influence from any earth electrode of a neighboring installation, otherwise earth faults on that neighboring installation could cause it to trip (by way of the bonded pipework on the "F" side of the coil then acting as the true earth reference).
The point surely is that by limiting the pd between the systems CPCs and 'earth' to ~50V, an CORLCB provided protection to someone who simultaneously touched a CPC and 'true earth' (i.e. an unbonded extraneous conductive part which, at least today, should not exist). However, for the reasons I explained in my previous post, in the presence of low resistance parallel paths from CPCs to true earth (e.g. via bonded pipework), such a device offered little protection to someone whose body became an L-'E' path (i.e. the sort of incident which RCDs offer some protection against).
Kind Regards, John
Kind Regards, John
VOELCBs were never intended to provide supplementary protection against direct contact, only 30mA and below COELCBs (RCDs) can be used for this.
(sorry if I used a 16th edn term)
It is also worth noting that VOELCBs placed a resistance in line between E and F, iirc somewhere around 160 ohms hence a majority of current will probably flow via the parallel paths to earth. It is just the touch voltage between the MET and mother earth which is monitored, the max touch voltage being 50v (or in some special locations it was 25v in the 16th edn).
(sorry if I used a 16th edn term)
It is also worth noting that VOELCBs placed a resistance in line between E and F, iirc somewhere around 160 ohms hence a majority of current will probably flow via the parallel paths to earth. It is just the touch voltage between the MET and mother earth which is monitored, the max touch voltage being 50v (or in some special locations it was 25v in the 16th edn).
P
Paul_C
That was the maximum permissible voltage under the worst conditions, but in practice the actual tripping voltage was often considerably less. One of the popular Crabtree types had a 500-ohm coil which was designed to trip the mechanism at a coil current not exceeding 35mA. It was specified for use with an earth electrode of up to 500 ohms. There was obviously a fair margin for electrode resistance increase built into that calculation, since even at that earth resistance the ELCB would trip at 35V maximum. If it was used with an electrode with, say, a resistance of a more modest 200 ohms, then the tripping point would be reached a little below 25V.
So long as that reference electrode was maintained at true earth potential by being completely outside the influence of anything else, then the ELCB would always do its job. Paths to earth on the "F" side of the coil would increase the earth current which would need to flow in order to raise the potential across the coil to the trip value, but unlike the current-operated ELCB the device was never designed to trip on the current exceeding a certain level, only the voltage, hence voltage-operated ELCB.
Certainly, but remember that the voltage-operated ELCB was never designed to try and protect against such things. It's sole purpose was to prevent the internal earthing system of the installation from rising to a dangerously high potential in the event of a fault which could not be cleared by regular fuses or circuit breakers due to the relatively high loop impedance.
In the heyday of the voltage-operated ELCB, there were normally no earth-leakage breakers of any type used in homes wired TN-S or TN-C-S.
- Joined
- 11 Jan 2004
- Messages
- 46,050
- Reaction score
- 3,581
- Country
But VOELCB's were deleted from the regs in 1985.
Sure, but did they not have ('main') bonding in those days? Provided one had an equipotential zone, the one thing these devices were designed to protect against was something which shouldn't happen, or be a risk to human beings - there's no risk (to human beings) in the internal earthing system of an installation (i.e. CPCs and exposed-conductive parts) rising to a 'dangerously high potential' relative to true earth if true earth potential does not exist within the building.
Kind Regards, John.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
