Supply Earths and their differences !!!

[Invisible-Man]

Hi,
could anyone please explain the differences between the differnet types of supply earth and the importants of designing your electric wiring instalation around this issue, ie, consumer unit choice etc.

Also is it dangerous (or how dangerous) to work on a circuit that is only disconnected via a single pole switch, leaving the negative connected, or another example is if the DNO main fuse on a supply was out would the negative side be safe to touch as it would still be connected!

Many thanks

 

[ricicle]

Why do you want to know?

 

[Invisible-Man]

Hi ricicle,

I have for years dabbled with electrics but out of interest of the subject and on reading alot of the posts do not quite understand the points mentioned above and was looking for some clarification

 

[Rusty2]

In my experience if you only disconnect the live via the switch then you will cause a short when you cut through or touch the negative with the earth. Better to disconnect the earth as well, or negative, ands then you can do what you like on the circuit without tripping anything out

 

[Lectrician]

No such thing as a negative on an AC system - a NEUTRAL, yes!

When working on any system, Double Pole isolation is prefered. However, aslong as the neutral is connected to neutral, there is little or no danger. 3 phase circuits, suprisingly, don't require the isolation of the neutral. On 3 phase circuits, I still try to fit 4 pole when possible - isolators are available on 63amp and below in 4 pole in the same price range as 3 pole.

With RCD's, you will suffer from RCD's tripping on a split load board / main RCD incommer if the other circuits supplied by the same device are drawing a current - this is because as you cut the cable, you short N to E, allowing a small amount (30mA or above on a 30mA RCD) to flow from the neutral to earth. This isn't *really* dangerous - they both return to the tranny star point eventually. This is when Double Pole isolation is a god send! No nuisence tripping then when working on circuits.

 

[ricicle]

Not sure I like the word dabbling but anyway:-

There are some diagrams explaining earthing arrangements in the Wiki,but to clarify TT system is where the DNO provide no earth and therefore an earth rod or spike is used and the ground is the path back to the transformer 'star' point.Depending on the type of ground that the rod is in will depend on the impedance of that path.This path is usually of a higher impedance than a direct 'cable' path and therefore all circuits in the installation have to be protected by an RCD (100mA fixed equip/30mA socket outlets etc) to ensure disconnection times are met.These systems are usually found in rural locations.
TN-S systems are where the earth is a seperate conductor all the way back to the 'star' point, this could be a core of the cable or the cable sheath.
TN-C-S (PME) systems use the seperate earth/neutral conductors in the consumers installation from the cut-out onwards but the earth is bonded to the neutral in the cut-out and are the same conductor back to the 'star' point.Certain criteria have to be met in order for this system to be used in some locations and it cannot be used in others.
Both TN-S and PME systems should ensure (if all connections etc ok) that a fairly low earth impedance exists, so disconnection times can be met on certain circuits without the need for an RCD using just MCB's/fuses.
As for working on circuits with neutrals not isolated if you look in the Regs they are described as LIVE conductors because they form part of the current carrying circuit.They are in theory at earth potential but there are a number of circumstances where they can be at live potential as well (borrowed neutrals, burnt off neutrals when the circuits are still live and is through the load) so NEVER assume anything is dead.
RCD's can trip out when individual MCBs/fuses are out due to other loads being on and the other neutral currents flowing out to earth in a temporary short caused by cutting through a cable with its neutral still connected.This imbalance (depending on its size) could cause the RCD to trip.
Hope this helps a little

 

[ricicle]

Damn you Lectrician

 

[plugwash]

3 phase circuits, suprisingly, don't require the isolation of the neutral.

i'd imagine there are two reasons for that, the first is that balanced load will tend to keep the voltage on the netral down, the second is a live-neutral swap in a 3 phase system will be noticed very quickly

With RCD's, you will suffer from RCD's tripping on a split load board / main RCD incommer if the other circuits supplied by the same device are drawing a current

all it takes is a potential between neutral and earth to cause this, the load that causes this potential does not have to be on the RCD or even in your property (unless you are TN-C-S in which case it does have to be in your property because neutral and earth are tied at your incomer).

This is when Double Pole isolation is a god send! No nuisence tripping then when working on circuits.

yep, its a pain having to tape up neutral conductors before removing socket boxes etc and to be very carefull how you strip cable just to avoid tripping a bloody rcd.

 

[Lectrician]

[code:1]all it takes is a potential between neutral and earth to cause this, the load that causes this potential does not have to be on the RCD or even in your property (unless you are TN-C-S in which case it does have to be in your property because neutral and earth are tied at your incomer).[/code:1]

I think this only rings true with DC sense types?

 

[Spark123]

Not really, if there is a potential between neutral and earth then current can flow, if this current flows through an RCD i.e. from a N-E short on a socket circuit then the RCD will trip. The current will almost certainly be AC as it is a result of volt drop on the neutral conductor.
The difference with TN-CS is with the neutral-earth tie in the property hence any potential between neutral and earth has to be created inside the property.

 

[Lectrician]

Just because there is apotential doesn't mean current will flow does it.

We have a potential of 230volt at a socket outlet, but current will only flow when you introduce a load.

 

[Invisible-Man]

Thanks for all your informative posts, it has helped me to understand some of the areas that I have not studied up to now.

Many thanks !!!

 

[Spark123]

Just because there is apotential doesn't mean current will flow does it.

I agree, there must be a path for current to flow.
My post was in the context of when a short from N-E is introduced as on a circuit as in a reply to your reply to Plugwash. It doesn't matter if the potential between neutral and earth is caused inside or outside the premises (unless it is a TN-CS where it must caused inside the premises). If there is a potential difference between neutral to earth and they are shorted together, a current will flow in the earth conductor (as in a parallel path). This current, if large enough will trip an RCD if the N-E short is on an RCD protected circuit.