Earth Bonding

[bernardgreen]

The thing to remember is that pipes can carry hazardous voltages and or currents into a property and out of a property.

Also accept that true ground potential imported into a building can be hazardous if the safety "earth" of the electrical system is not at true ground. As can easily happen since in many installations the safety earth is in reality the same potential as the incoming neutral of the power supply.

Hence sparks and current flow when a metallic service pipe is connected to the safety "earth" of the installation.

That said on balance and for the vast majority of homes the system does work and is safe. The few exceptions, such as metallic service pipes providing low impedence connections to true ground, do create problems.

 

[sparkticus]

('another' meaning a property {third or subsequent} other than the one with a lost neutral and the one without a lost neutral in which the pipe was cut.) In terms of your diagram, if point C was connected to supply neutral via metal pipework and the bonding of a third property which had not lost it's neutral, then the PD between A and B ought to be pretty low (just the voltage drop along the length of pipework + bonding) ... isn't that the case?

Yes I would agree with that. The third property would essentially "act" in place of the original connection (broken neutral) There would be some voltage drop due to resistance. This highlights many hidden dangers of what would otherwise be thought of as electrically "inert" structures within the home. Another classic misunderstanding is "it's only an earth wire, that can't hurt you" (possibly true provided you don't hang on the end of it while disconnecting)

As a matter of personal interest does anyone agree with my scenario above (earlier in this thread) reproduced below, or have I misunderstood something:

Just one possible example: (I think)

Two houses next to one another House A - House B - TT earthing system.


1) House A: TT spike is fine with a nice low Ra.
2) House B: TT spike is missing/disconnected
3) Lead water main serves house A & B
4) Plastic main run to lead at the pavement position.
5) House B has a "leaky" immersion heater and no RCD in circuit.
6) House A receives the current from house B on its lead water pipe.
7) In house A Mr Smith cuts into the pipe and now there is (or could be) a significant PD across the two ends of the pipe with the higher end originating from house B

 

[JohnW2]

Also accept that true ground potential imported into a building can be hazardous if the safety "earth" of the electrical system is not at true ground. As can easily happen since in many installations the safety earth is in reality the same potential as the incoming neutral of the power supply. Hence sparks and current flow when a metallic service pipe is connected to the safety "earth" of the installation.

You have surely just given the explanation as to why we have main protective bonding?

Given that the path to true earth through the e-c-ps (e.g. incoming water supply pipes) will be far from zero, in practice the potential of everything in the premises will rise (to above the potential of true earth) if the potential of the TN "earth" is above that of true earth - but with bonding in place, there will be no significant PDs within the premises.

Kind Regards, John

 

[JohnW2]

Yes I would agree with that. The third property would essentially "act" in place of the original connection (broken neutral)

Exactly.

As a matter of personal interest does anyone agree with my scenario above (earlier in this thread) reproduced below, or have I misunderstood something:
Just one possible example: (I think) .... Two houses next to one another House A - House B - TT earthing system.
1) House A: TT spike is fine with a nice low Ra.
2) House B: TT spike is missing/disconnected
3) Lead water main serves house A & B
4) Plastic main run to lead at the pavement position.
5) House B has a "leaky" immersion heater and no RCD in circuit.
6) House A receives the current from house B on its lead water pipe.
7) In house A Mr Smith cuts into the pipe and now there is (or could be) a significant PD across the two ends of the pipe with the higher end originating from house B

Yes, FWIW I agree (assuming that both houses had their installations bonded to the lead water main). However, there are other hazards present here, essentially because of the lack of RCD protection in house B ...

.... a similar hazard to that which you describe would presumably exist even if House B's TT spike was present and correct. With no RCD, the 'leaky' immersion heater would almost certainly not cause an MCB to operate (you'd be hard pressed to get much more than about 5A through the average TT electrode), so the potential of the CPCs and pipework (including the lead supply pipe) in house A (and also house B, so long as pipe connection existed) would still rise to appreciably above true earth - hence again creating a potentially hazardous PD between the cuts ends of the pipe (whether cut in house A or house B). Nor would an RCD in house A help at all - it's one of those situations in which only a 'voltage-operated' protective device would be of any use.

Kind Regards, John.

 

[sparkticus]

Also accept that true ground potential imported into a building can be hazardous if the safety "earth" of the electrical system is not at true ground.

Yes absolutely, no question. It's all really about attaining an "equipotential zone" which is not always easy and sooooo many times misunderstood by even experienced electricians. I have seen the work of electricians who instead of running a main protective conductor from the MET to a gas main (because its a long distance and lots of work) Decide to spike the gas meter (I mean sink a rod and connect to the gas meter) This shows that they simply do not understand the actual reason for bonding everything together.

 

[JohnW2]

I have seen the work of electricians who instead of running a main protective conductor from the MET to a gas main (because its a long distance and lots of work) Decide to spike the gas meter (I mean sink a rod and connect to the gas meter) This shows that they simply do not understand the actual reason for bonding everything together.

I think much of the problem is a failure of basic education about the principles. I think that, unless educated to 'know better', many people (including some of those in relevant trades) do not understand that connections to 'the general mass of the earth' have finite, often appreciable, impedence. By not realising that, they 'understandably' assume that all things 'connected to earth' are effectively connected together - as if our planet were a sphere of copper! Hence, even if they understand the concept of bonding and equipotential zones, they may still think that two things 'connected to earth' (but different 'earths') are inevitably equipotential.

Kind Regards, John.

 

[sparkticus]

I think much of the problem is a failure of basic education about the principles. I think that, unless educated to 'know better', many people (including some of those in relevant trades) do not understand that connections to 'the general mass of the earth' have finite, often appreciable, impedence. By not realising that, they 'understandably' assume that all things 'connected to earth' are effectively connected together - as if our planet were a sphere of copper! Hence, even if they understand the concept of bonding and equipotential zones, they may still think that two things 'connected to earth' (but different 'earths') are inevitably equipotential.

Absolutely a lack of understanding of the principles. Yet if I feel that I don't understand something, and there are many such things even after 35+ years in electronics/electrical business, I will research it or ask someone who does. That is all I expect from others in the trade/profession yet you get people doing the most ridiculous things (often with hazardous consequences) without thinking it through. I encourage new entrants to the business to ensure they understand the basic principles first (have they thought through the basic science of what they are about to do) at which point (and I think I have said this before) you see their eyes glaze over. One young person told me that I (me) always have to complicate things, after all its just "a bit of cross bonding" Arggggggg!!

 

[bernardgreen]

Yesterday morning insulation on data cable connecting a computer in a house and another computer in a workshop 200 yards was found part melted. The woven screen in the cable appears to have become very hot. The house and workshop are fed from different network circuits. House underground from the street and workshop from a pole mounted transformer on a different part of the network. Screen connected via computers to CPC ( electrical "earth") at both ends. No RCDs were tripped.

Immediate fix is to install opto isolator module in the link. ( should have been there from the beginning ) The only logical cause would be a "bounce" on the electrical safety earth at one end of the link and a subsequent heavy fault current along the screen.

 

[davelx]

Yesterday morning insulation on data cable connecting a computer in a house and another computer in a workshop 200 yards was found part melted. The woven screen in the cable appears to have become very hot. The house and workshop are fed from different network circuits. House underground from the street and workshop from a pole mounted transformer on a different part of the network. Screen connected via computers to CPC ( electrical "earth") at both ends. No RCDs were tripped.

Immediate fix is to install opto isolator module in the link. ( should have been there from the beginning ) The only logical cause would be a "bounce" on the electrical safety earth at one end of the link and a subsequent heavy fault current along the screen.

You should ideally plan to replace the cable with fibre - which is what really should have been installed from the start!

I've seen Cat5 & ThinWire used between buildings and once even the original 10base5 "thick" Ethernet with bee-sting taps strung between trees before now, with the drop cables entering the huts via the windows. (this was on an RAF base!). No-one had checked what current was flowing in the screens, but not enough to cause any problems - more by luck than judgement!

 

[bernardgreen]

You should ideally plan to replace the cable with fibre - which is what really should have been installed from the start!

The link was installed long before fibre became economical. For the application RS485 ( now with opto isolation ) is more than adequate.

An update is that a large industrial drier fed from the same transformer popped a phase over the weekend and ran for several hours on just two.