jxofaltrds
Inspector Mike®
- Location
- Mike P. Columbus Ohio
- Occupation
- ESI, PI, RBO
Ground wire and Equipment Grounding Conductor
- Thread starter nkalghatgi
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glennspark
Member
- Location
- Leeds
i`m not on about the bloody connections...
i`m on about the purpose it serves....and why so called `electricians` persistantly confuse bonding (either main or supplementary) with earthing...or what you call grounding...
the amount of times i`v heard folk refer to such stuff as `earth bonding`....it dont exist and it demonstrates a clear lack of understanding on what earthing...and bonding is...and the roles they play......
jxofaltrds
Inspector Mike®
- Location
- Mike P. Columbus Ohio
- Occupation
- ESI, PI, RBO
Maybe I am missing your point. What I am talking about is the purpose as I understand it.
Effective Ground-Fault Current Path. An intentionally
constructed, low-impedance electrically conductive path designed
and intended to carry current under ground-fault conditions
from the point of a ground fault on a wiring system to
the electrical supply source and that facilitates the operation of
the overcurrent protective device or ground-fault detectors on
high-impedance grounded systems.
glennspark
Member
- Location
- Leeds
whats this got to do with bonding?
- Location
- Illinois
- Occupation
- retired electrician
Probably because the NEC calls what is really a bonding conductor the "equipment grounding conductor".
It prevents the situation you mentioned in the first place. You mentioned crossbonding two "parts" to prevent a difference in potential between them. Our equipment grounding conductor, which is really a bonding conductor, connects conductive parts that aren't supposed to be energized to the return of the power source. The return side of the power source is the grounded side, so that's why the confusing "grounding conductor" is used. So if that metal part gets energized, the power source is shorted out which trips the overcurrent device. We usually bond each item that isn't supposed to be energized (e.g. a metal electrical enclosure). We don't typically bond separately from one enclosure to another, because each one is safed buy the equipment grounding/bonding conductor in the power feed to that enclosure.
Some situations in the NEC may require equipotential bonding grids or "cross" bonding conductors (e.g. swimming pool and hot tub decks and all metal parts within 5' of them).
glennspark
Member
- Location
- Leeds
lol...
its caus where working to different regs mate....you work to the NEC....i work to BS7671
but the fact remains that there seems to be confusion between earthing and bonding conductors here....
the interconnections between earth rods to create a `grid` or `nest` is all about parrallel paths...
bonding is there to keep extranious conductive parts at a similar potential.....
extranious conductive parts are parts that do not form part of the electrical installation....but could provide a path to earth under fault conditions....
so it seems to me that theres some confusion thinking that the bonding is what we refer to in the UK as the CPC...circuit protective conductor...when in fact bonding is a totaly seperate issue...
to put it in context:
earthing limits the duration of a touch voltage
bonding limits the value of a touch voltage
jxofaltrds
Inspector Mike®
- Location
- Mike P. Columbus Ohio
- Occupation
- ESI, PI, RBO
I should have welcome to the forum so welcome to the forum.
For the most part what we discuss here is the NEC not what happens on the other side of the pond.
Please help this non-engineer understand what you mean by what I highlighted.
I think that you have them reversed but please still explain.
- Location
- Illinois
- Occupation
- retired electrician
Your CPC is our Equipment Grounding Conductor. In both cases the function is to bond the non-current carrying conductive parts back to the system grounded conductor to provide a fault clearing path.
glennspark
Member
- Location
- Leeds
lol...nope...right way round mate...
earthing is to provide a fault path of low enough impedence for a disconnection time to be met by any OPD (overcurrent protective device) protecting that circuit....its what we refer to as ADS in the UK..Automatic Disconnection of Supply....this includes fuses, breakers, RCDs (GFCIs) and RCBOs...
hence the statement i made about earthing limiting the duration of a touch voltage
if several pieces of extranious metalwork are bonded...there isn`t the potential difference between them....thus if say a touch voltage of 230V was on both...and you came into contact with them both simultaniously....you would get the difference between them....which would be 0V...
hence my statement about bonding limiting the value of touch voltage...
glennspark
Member
- Location
- Leeds
someone (a friend in the UK) had kindly sent me copies of the codes= NEC 2002, NEC 2008 and the NEC 2011....i shall have to have a look through it....see what similarities there are with BS7671 2008 1st..... (there are bound to be many). Actually i must admit i have had a little look through and there is a lot of differences in terminology....
You can't compare your distribution systems to ours, as there are a lot of differences. For the US, low voltage circuits (120V relative to ground) connected to earth will not trip our smallest (15A) overcurrent device in many cases (e.g. residential, where there is no building steel or other decent electrode). The electrode resistance to earth is too high. If you have an RCD or other leakage device that will trip at well under 1 amp, then earthing will provide that protection. We don't have RCDs on most of our circuits, so the fault/touch voltage duration would be infinite if the earth was the only path for fault clearing.
Your starting voltage to earth is double ours (240 -vs- 120). But even that or our next step up of 277V to earth most likely won't trip a 15A breaker on a earth electrode fault.
Connecting to earth is for systems or surges of much higher voltage, and to dissipate static charge build up.
The situation you keep describing (running a conductor between various metal items) would be more typical in our high voltage substations. That, in addition to metal grids in the dirt, and other things all work well when the voltages are quite high (well over 1 KV). Connecting our residential distribution systems (120V) to earth doesn't do a lot for safety. Connecting the raceways and enclosures to the electrical return path does.
The sad thing is, we call our bonding conductor (fault clearing conductor) an equipment grounding conductor.
Your starting voltage to earth is double ours (240 -vs- 120). But even that or our next step up of 277V to earth most likely won't trip a 15A breaker on a earth electrode fault.
Connecting to earth is for systems or surges of much higher voltage, and to dissipate static charge build up.
The situation you keep describing (running a conductor between various metal items) would be more typical in our high voltage substations. That, in addition to metal grids in the dirt, and other things all work well when the voltages are quite high (well over 1 KV). Connecting our residential distribution systems (120V) to earth doesn't do a lot for safety. Connecting the raceways and enclosures to the electrical return path does.
The sad thing is, we call our bonding conductor (fault clearing conductor) an equipment grounding conductor.
glennspark
Member
- Location
- Leeds
so this would be metallic enclosures...such as dis-boards, busbar chambers and metal plate switches etc...
metallic enclosures can be used to provide a fault path (earthing)..this includes galv conduits and metal trunking etc...
we have it that main bonding is used for connecting extranious conductive parts back to the MET (main earthing terminal)...and supplementary bonding (crossbonding) is utilised to connect extranious conductive parts to each other...thus maintaining the equepotential zone...
glennspark
Member
- Location
- Leeds
indeed..
so do all systems in the US rely on TT?
we have TT in the UK and in fact for certain scenareos (what we call special locations in part 7 BS7671) TT is a requirement.
we also use TN-C-S (terra-neutral-combined-seperate)..the Max. Ze allowed for this is
.35 ohms ..this utilises the neutral as the fault return path back to the TX upto the intake at the consumers property...at which point the main earthing conductor breaks out of the neutral-earth block in the cutout..and into the system`s MET...
then theres also TN-S (terra-neutral-seperate)..this utilises the cable sheath as the earth fault path...such as paper-lead cables....there us usually a constant pressure spring at the cutout around the incoming cable sheath...where the main earthing conductor is connected...the Max value of Ze allowed for this system in the UK is .8 ohms.
now...lets take an 32A B curve MCB with characteristics to BSEN60898...to achieve a disconnection time of 0.4 secs requires that the value of Zs does not exceed 1.44 ohms....quite achievable with TN earthing ...a C curve has half the Max. Zs for time of a B curve..and a D curve has half the Max Zs for time of a C curve....C & D curve are for inductive loads where a B would see start up currents (infush) as `fault`...but the maz Zs decreases for each...so being able to acheave a low enough Zs for time is partly down to the earthing arrangements...and also to good design ...
in the case with TT... RCDs are considered `fault protection` for TT..as its often the only way disconnection times can be met...
RCDs for TN systems are `additional protection`....
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
What is even sadder is I still see many still not knowing the differences even if we had different names for the mentioned items.
We really need, but will never see in any of our lifetime, is to not use grounded conductors as normal current carrying conductors, and we will see a big decrease in potential objectionable current and so called "stray voltages" in components not intended to carry current. This even applies to POCO use of a MGN network.
glennspark
Member
- Location
- Leeds
but `stray voltage` is induced....from current carrying conductors in proximity...so you still need to dump it to earth...dont you...
stray voltage appeares where there is no reference point...the further away from a reference point you are (such as the star point on a TX)...the more likely there will be a few volts potential between conductors that are both referenced at the starpoint.....if this makes sense...
- Location
- Illinois
- Occupation
- retired electrician
The US uses TN-C-S.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Yes and no. Most of the "stray voltage" and I put quotations around that for a reason, I am talking about is what many call voltage phenomena that they really don't understand. It could be from current carrying conductors in proximity to something, but for most NEC applications this doesn't happen because of the requirement in 300.3(B) which says:
Following that will ensure that magnetic fields around the conductors of a particular circuit will cancel one another limiting any induced current to other objects. So IMO that source of stray current is not much of a problem most of the time. What seems to be more easily overlooked is parallel paths for intentionally grounded current carrying circuit conductors to take, that is what makes up most of the "stray voltage" I am talking about and what seems to cause the most trouble and confusion. Any interconnection between those grounded current carrying conductors and other grounded objects beyond a single earthing point (generally at/near the source) are subject to introduce problems.
jxofaltrds
Inspector Mike®
- Location
- Mike P. Columbus Ohio
- Occupation
- ESI, PI, RBO
Excuse my non-engineering background. They only thing that "dump(s) to earth" is lightning. Everything else is trying to get back to 'it's' source. Correct?
Not sure about sprites.
http://www.bing.com/videos/search?q=Lightning+Sprites&FORM=RESTAB#view=detail&mid=198F8E6C8190E35B7420198F8E6C8190E35B7420
glennspark
Member
- Location
- Leeds
i think that quotation may be to do with what we call `eddy current`...which can be induced in metallic enclosures where cables are using different entrys....for example one knockout is used for the phase...and another knockout is used for neutral...
i have come across this one quite a bit when carrying out condition reporting in the UK...its unqualified, incompetant inexperienced guys thats doing it...and it can cause enclosures to overheat due to recirculating currents...which can be nasty.
the solution:
1) make sure that all current carrying conductors of a circuit enter an enclosure through the same entry point
2) use a non ferrous bushing for each entry
3) cut slots between entrys to disrupt the eddy (this risks compromising the IP (ingress protection) of an enclosure though...
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