steel conduit-separate earth

davelerave

a large job wired in steel conduit/trunking has all separate pvc cpc's(1.5,2.5) for lights ,sockets etc
is there a reason for this,am i missing something obvious thanks

eoghan.geraghty

Each circuit must have it's own cpc, that's all there is to it.

davelerave

for steel conduit?

eoghan.geraghty

Yes, I know back in the days of yore the steel conduit satisfied as the cpc but not anymore, since I started in 97 at least.
The conduit must still be earthed at any box with terminations too.

davelerave

haven't been on steel for years but you sure.
wiring rules 543.2.1 543.2.4

Rev. BlueJeans

AFAIK, and I'm no regs expert, all boxes have to be bonded to earth, whether connected by steel conduit or not.

This has been customary on any installations I've seen anyway, FWIW.

davelerave

Rev. BlueJeans wrote: »

AFAIK, and I'm no regs expert, all boxes have to be bonded to earth, whether connected by steel conduit or not.

This has been customary on any installations I've seen anyway, FWIW.

Just querying the use of separate earth in a steel system

543.2.1: Protective conductors may consist of any of the following:Metal conduit,trunking and other metal enclosures of conductors.

I can understand 'supplementary' where continuity isn't assured

eoghan.geraghty

It appears you can subject to 543.2.4, but I've never known anyone to rely on the conduit and trunking as a cpc, it was done as earth bonding.
Remember you must keep the overall value of the cpc below 1 ohm.

I have seen it in older installations ( 1970s)and always recommended an upgrade.

I suppose in answer to your query, one might get away with relying on the conduit as a cpc but for the sake of pulling in an earth why would you?

davelerave

i'm amazed if steel requires separate cpc in normal use:D

worked on steel/industrial for years before the boom ,never saw a separate cpc .

eoghan.geraghty

I did only steel conduit,tray and trunking jobs from 97 to 02 and have barely touched it since, though I did a small conduit job last year and had to splash out on a bending machine:mad:

If you're adding to the job would you use a new earth or rely on the steel? Just being nosey now;)

davelerave

i'd like to find out what i'm doing first:D

2011

Each circuit must have it's own cpc, that's all there is to it.

The CPC can be steel conduit according to the ET101. Even a steel trunking and steel combination is permitted.

However the installer must ensure that the earth fault loop impedance is not too high.

a large job wired in steel conduit/trunking has all separate pvc cpc's(1.5,2.5) for lights ,sockets etc

This is standard practice for most industrial installations. There can be several reasons for this:

1) The design engineers have specified that this is required. They will often have requirements that are above and beyond what the regulations require. I have worked on projects where a maximum of only 6 lights per circuit is allowed, this offers greater flexability and to allow for future expansion etc.

2) Using steel conduit as a CPC can be a problem if the resistance of the conduit creeps up over time. This can happen due to corrosion, or perhaps bushings work loose due to vibration. Sometimes conduit can suffer mechanical damage that may not be apparent. This is why it is emphasised that all steel conduit should be tightened thoroughly ensuring that all threaded joints present low resistance connections.

Normally in industrial installations socket circuits would be wired in 4 sq. cable with a 2.5 sq. CPC (at leat 1/2 the size of the phase conductor). However on some projects (e.g. parts of Dublin Airport) the electrical design engineers specified that the CPC must be the same size as the phase conductor, despite the fact that this requires the use of a 4 sq. earth for a socket circuit! This also explains why so many of these projects use MK sockets, the connections are large enough to be able to take two 4 sq. cables.

I can understand 'supplementary' where continuity isn't assured

I have seen cases where steel conduit is not continious such as where a circuit passes through a mass concrete wall and is picked up through an "end box" on each side of the wall.

all boxes have to be bonded to earth, whether connected by steel conduit or not.

If the boxes are threaded to steel conduit that is connected to earth at some point they do not all need to be individually bonded. If a conduit box has something like a "click socket" or switch etc mounted to it then it would be bonded with a suitable cable.

2 stroke

davelerave wrote: »

a large job wired in steel conduit/trunking has all separate pvc cpc's(1.5,2.5) for lights ,sockets etc
is there a reason for this,am i missing something obvious thanks

Don't know the regs, but I can sugest a reason. I've seen this spec'd in hazardous areas, early 90's. Remember sparks commenting that he was lucky he hadn't quoted for materials, as all cables were way overspec in his opinion. (Poor fecker was killed pulling cable and I heard a lot of his opinions). Company is still there though.

davelerave

rules don't 'necessitate ' separate cpc, but as already said probs can arise with bushens etc.

must say i've never noticed 'high loop impedance' except for damp locations.

it's still only 'supplementary' though .over 90% of fault current will still take the 'steel return path' anyway.

Dotsie~tmp

davelerave wrote: »

haven't been on steel for years but you sure.
wiring rules 543.2.1 543.2.4

All separate earths now + all trunking joints copper bonded.

2011

All separate earths now + all trunking joints copper bonded.

The regulations clearly state (as shown above) that you are permitted to use the steel conduit as a CPC. Therefore a seperate earth is not required in all cases.


What you are refering to (I would guess) is what you are seeing, which I'd that design engineers are requiring it on many projects. This is quite different from it being an ETCI requirement.

Earth tags on metal trunking has always been a requirement.

davelerave

well seems to be 'standard practice' anyhow judging from feedback

2011

I agree Dave, but there is a difference between that and what the regulations require as a minimum.

"Good practice" and the "industry norm" is to run a seperate CPC.

Bruthal

2011 wrote: »

I agree Dave, but there is a difference between that and what the regulations require as a minimum.

"Good practice" and the "industry norm" is to run a seperate CPC.

Well i definitely think it would be bad practice to rely on steel conduit as circuit protection. Just because regulations dont require something does`t mean its up to a good standard just by meeting the regulation level.

M cebee

well i'm back on steel now after years away from it.

all separate cpc's alright

when did the change to separate earth come in.

steel was alright as cpc for years before .granted problems can arise with corrosion and joints etc.

davelerave

Bruthal

M cebee wrote: »

well i'm back on steel now after years away from it.

all separate cpc's alright

when did the change to separate earth come in.

steel was alright as cpc for years before .granted problems can arise with corrosion and joints etc.

davelerave

Well im doin it around 20 years and i always ran in seperate earths,

2011

Well i definitely think it would be bad practice to rely on steel conduit as circuit protection

I take your point Robbie, but there are many situations where a seperate CPC will make no difference whatsoever because the earth fault loop impedance is so low without it (after all steel is a very good conductor and even 20mm conduit has a large cross sectional area).

On a large project it is often the case that the job specification would specificaly require seperate CPCs. In that case all electrical contractors would be pricing on the same basis.

However on other jobs the requirement is often that the electrical installation must meet meet current standards (which to be fair are quite high). Due to ever decreasing margins electrical contractors are trying to do work to the required standard with the least amount spent on time and materials (the price of cable has more than doubled in the last few years and extra cables may can mean a larger conduit and trunking sizes). These are tough times:(

As a contractor you could always price it both ways and give the client the choice:)

When I worked on my tools on large projects we always ran a seperate earth, but that was always part of the spec. On small installations I have seen cases when this has not been done.

Anyway I agree that it is "good practice"

M cebee

i assume continuity on steel and earth wires should be checked separately
as there's no way of confirming when sockets etc are terminated

in normal use the separate earths are supplementary ' and steel will take majority of fault current

Bruthal

2011 wrote: »

I take your point Robbie, but there are many situations where a seperate CPC will make no difference whatsoever because the earth fault loop impedance is so low without it (after all steel is a very good conductor and even 20mm conduit has a large cross sectional area).

On a large project it is often the case that the job specification would specificaly require seperate CPCs. In that case all electrical contractors would be pricing on the same basis.

However on other jobs the requirement is often that the electrical installation must meet meet current standards (which to be fair are quite high). Due to ever decreasing margins electrical contractors are trying to do work to the required standard with the least amount spent on time and materials (the price of cable has more than doubled in the last few years and extra cables may can mean a larger conduit and trunking sizes). These are tough times:(

As a contractor you could always price it both ways and give the client the choice:)

When I worked on my tools on large projects we always ran a seperate earth, but that was always part of the spec. On small installations I have seen cases when this has not been done.

Anyway I agree that it is "good practice"

Well it may pass the earth loop test but that does not mean its a good solid fault path, if a bushing was only hand tight that would not be accepted as a good earth fault path, but might pass an earth loop impedence test. If a 63 amp isolator was installed and the earth was via the conduit and a phase to earth occured then the loose bushing might show up all black as it was not a good connection, even in a 20 amp socket circuit this could happen although the RCD would trip unless no earth path at all, but loose bushings would often be seen on installations. The same can be said of loose earth wires in the circuit but at least you can find the ends.

The current standard requirements may be high but that does`t mean the actuall work carried out is. Bad standards can still pass electrical tests. By including circuit earths at least it reduces the scope for bad installation work. Im actually surprised it is still within regs to use the conduit as the protection. And while the earth conductor is in parallel with the steel as you suggest by saying the readings will be the exact same with or without the earth wire, that would be true if you ran in a 1.5 earth for a 16 square circuit, but still is not within regs to do so.

2011

robbie7730 wrote: »

Well it may pass the earth loop test but that does not mean its a good solid fault path, if a bushing was only hand tight that would not be accepted as a good earth fault path, but might pass an earth loop impedence test.

The issue of loose bushings was addressed in an earlier post. Yes I agree loose bushings are not good, but neither are any loose electrical connections (phase, neutral or earth).

If a 63 amp isolator was installed and the earth was via the conduit and a phase to earth occured then the loose bushing might show up all black as it was not a good connection

Same with a loose cable connection

even in a 20 amp socket circuit this could happen.....

Yes it could happen with any circuit with any type of earth path, conduit, cable, metal frame etc...

The same can be said of loose earth wires in the circuit but at least you can find the ends.

Yes you may find the ends or the loose connection, but it may be too late then and someone may be hurt/dead and a building might have gone on fire!!!

The current standard requirements may be high but that does`t mean the actuall work carried out is[/QUOTE]
I agree 100%

By including circuit earths at least it reduces the scope for bad installation work

I agree. But if a good electrician does the job properly a good earth path can be provided by steel conduit.

Im actually surprised it is still within regs to use the conduit as the protection

When done correctly it is fine. It is not suitable in all situations.

while the earth conductor is in parallel with the steel as you suggest by saying the readings will be the exact same with or without the earth wire......

Not in all cases, bu tin some. It depends on how many couplers/joins, how long the conduit run is etc...

I agree with you that a seperate CPC is better practice, my point is that times are tough and in some situations aseperate CPC makes no difference.
Have a nice day!!!:):):)

Stoner

you also have the issue with people breaking the links with some conduits and trunking, also on top of that most jobs with a decent bit of metal work will have to follow a particular and general specification of a consultant engineering company, most of these specs will insist on a dedicated earth for each circuit.

It's just a no no these days really, shows up in dilapidation reports etc as a failure of existing installation to meet the current regs.
Regardless of what the current regs say, clients pay money to correct it, contractors have their payments held up they use the tray and conduit as an earth on contracts.
If I saw it on a job I hold up up there with an unmarked switchboard as a sign of a potentially poor job, just as an indicator if you know where I'm coming from.

With respect to earthing I've never seen a backward step, it's always more bonding, larger CPCs etc.

Bruthal

2011 wrote: »

I agree with you that a seperate CPC is better practice, my point is that times are tough and in some situations aseperate CPC makes no difference.
Have a nice day!!!:):):)

Yes well seperate earth conductor is the proper way in my opinion, loose bushings break the path, loose earth connections also, but which would you prefer to address, finding a loose earth connection in a socket or at an mcb board, or try to find a loose bushing in an attic or ceiling space, try to find which one it is, and better still try to tighten it by removing the lid and the bushing now full of wires, assuming you ever find it. If bad times recession wise means cutting back on earth wires then i think its beyond hope. Just because REG`s allow conduit as an earth does not mean its right, REG`s are not infallible. if it is changed then will it suddenly be wrong? I always thought ring mains were bad, they were allowed, not sure if they are allowed now or went last sept. But i really believe the day of steel containment being the earth is past now.

Also i always thought it was required to earth the steel back box on sockets even if a surface box on end of steel conduit. It seems using conduit would be backwards to this, as it would be earthing the socket from the bac box instead. All this is just my very small opinion. But i would`t wire any circuits using the conduit as earth. It does`t mean im right. But at the end of the day it is far easier to get a good connection at the end of an earth wire than it is depending on multiple trunking and conduit joints. And even easier to find the problem if there is a bad connection. And it also helps the integrity of the earthing all round as the circuit protective conductors are earthing the conduits again via earthing of back boxes etc.

Bruthal

Stoner wrote: »

you also have the issue with people breaking the links with some conduits and trunking, also on top of that most jobs with a decent bit of metal work will have to follow a particular and general specification of a consultant engineering company, most of these specs will insist on a dedicated earth for each circuit.

It's just a no no these days really, shows up in dilapidation reports etc as a failure of existing installation to meet the current regs.
Regardless of what the current regs say, clients pay money to correct it, contractors have their payments held up they use the tray and conduit as an earth on contracts.
If I saw it on a job I hold up up there with an unmarked switchboard as a sign of a potentially poor job, just as an indicator if you know where I'm coming from.

With respect to earthing I've never seen a backward step, it's always more bonding, larger CPCs etc.

Forward steps for certain, i actually did`t realise it was within regs not to use a dedicated circuit earth, but knowing it is i still would`t do it without them.

Bruthal

2011 wrote: »

Yes you may find the ends or the loose connection, but it may be too late then and someone may be hurt/dead and a building might have gone on fire!!!

Well if the cpc being loose is that dangerous where would it be if depending purely on the conduit as earth.

And the big question,,, would you wire a circuit without the earth if it was up to yourself on a job?

And have you ever seen a bushing thats loose into a trunking. I have. And i seen loose earths in terminals. And also the conduit used as earth still requires an earth wire to be connected to the terminal of the outlet, so the loose terminal possibility applies anyway if just using steelwork as earth. So a loose earth cpc cant really be used as an arguement against using a dedicated earth.

pdiddy

in most buildings with steel trunking conduit and tray they are still used as cpc's because most back boxs are earthed and also trunking is normally earthed where it joins into the main panel but a seperate earth wire is also used therefore you actually have two earth paths one through the conduit etc and the other through the earth cable,thus reducing the risk of one or the other being faulty due to loose bushings or loose connections

Bruthal

pdiddy wrote: »

in most buildings with steel trunking conduit and tray they are still used as cpc's because most back boxs are earthed and also trunking is normally earthed where it joins into the main panel but a seperate earth wire is also used therefore you actually have two earth paths one through the conduit etc and the other through the earth cable,thus reducing the risk of one or the other being faulty due to loose bushings or loose connections

Well id say its more that the the earth wire in the circuit is used as the cpc, and the trunking and conduit is earthed and bonded together to prevent it becoming live in a fault condition by operating protective devices. But its neither here nor there, maybe some would be happy to wire 3 phases and neutral to an isolator and save on the earth because of the recession. I would`t like to though. thats just me, i seen plenty of stuff done that i would`t do.