equipotntial bonding

Robbie71

trying to understand this
if all metal pipes etc are bonded and say there is a fault and all the pipes are now at 230v ....

if someone now touches a "live" pipe will they get a shock as they are standing on floor (earth)?

is the EP bonding cable connected to the earth cable in an electric circuit?

RoundyMooney

Robbie71 wrote: »

trying to understand this
if all metal pipes etc are bonded and say there is a fault and all the pipes are now at 230v ....

if someone now touches a "live" pipe will they get a shock as they are standing on floor (earth)?

is the EP bonding cable connected to the earth cable in an electric circuit?

It certainly is-and also (in the majority of cases) to the incoming neutral conductor as well, which is itself backed up by further, and more comprehensive earthing.

Assuming a broken neutral, the body of metal within the premises or installation will rise to above earth potential, depending on a number of factors. Once the bonding remains intact, the risk of shock from metal objects at two different potentials is minimised.

Bruthal

Robbie71 wrote: »

if someone now touches a "live" pipe will they get a shock as they are standing on floor (earth)?

Usually in dry conditions there will be little or no shock in that scenario. Bare feet on tiles downstairs might give a slight one, but a dangerous level one would be unlikely.

With shoes or runners on, a shock would also be unlikely between a live pipe and the floor.

And as said, with all metal connected to the earth through bonding, there should be no potential difference between metal items, which helps minimise shock risk.

Merch

if there is an earth fault, could that not cause a main breaker to trip (or main fuse to blow).
Assuming the device with the fault isn't an appliance which had its own fuse.

Bruthal

Merch wrote: »

if there is an earth fault, could that not cause a main breaker to trip (or main fuse to blow).
Assuming the device with the fault isn't an appliance which had its own fuse.

It will, or at least should. But the situation here where the metal goes up toward 230v would be due to the neutral failing upstream of the meter, so leaving an open neutral, with the earthing system of the house connected to it (neutralised system).

So there is no earth fault. In theory the earth rod would keep the potential down, but to what level depends on the load thats on in the house when the neutral fails, and how good the earth rod is. If it fails inside the house after the meter and neutralising connection, there is no such problem with metal going above ground potential.

Merch

robbie7730 wrote: »

It will, or at least should. But the situation here where the metal goes up toward 230v would be due to the neutral failing upstream of the meter, so leaving an open neutral, with the earthing system of the house connected to it (neutralised system).

So there is no earth fault. In theory the earth rod would keep the potential down, but to what level depends on the load thats on in the house when the neutral fails, and how good the earth rod is. If it fails inside the house after the meter and neutralising connection, there is no such problem with metal going above ground potential.

An open neutral, you mean a damaged/disconnected neutral (possibly upstream of the meter), is called open neutral?

I was just going by the OP but this is as curious

Bruthal

Merch wrote: »

An open neutral, you mean a damaged/disconnected neutral (possibly upstream of the meter), is called open neutral?

I was just going by the OP but this is as curious

Yea I guess he is actually saying a fault. But the fault that causes such a scenario as mentioned is a neutral failure before or right at the meter cutout.

Failed neutral = broken neutral or open neutral. Usually caused by failure of a connection.

Armin_Tamzarian

Merch wrote: »

if there is an earth fault, could that not cause a main breaker to trip (or main fuse to blow).
Assuming the device with the fault isn't an appliance which had its own fuse.

That's the point of earthing, the excessive current flowing to earth causes the overcurrent device to trip, i.e. the breaker/fuse.
If there was no decent earth path the level of fault current might remain too low to trip the breaker or blow the fuse.
Even if the device in question doesn't have it's own fuse, the breaker feeding that circuit will trip.

I think the original question goes back to the common enough lack of understanding that people won't get shocked if their body isn't in contact with earth.

Bruthal

Armin_Tamzarian wrote: »

That's the point of earthing, the excessive current flowing to earth causes the overcurrent device to trip, i.e. the breaker/fuse.
If there was no decent earth path the level of fault current might remain too low to trip the breaker or blow the fuse.
Even if the device in question doesn't have it's own fuse, the breaker feeding that circuit will trip.

The main point of equipotential bonding is to ensure all metal remains at equal potential during fault conditions. The most likely fault it protects against in neutralised systems is a failed neutral.

The provision of the decent earth path for earth faults is actually via the neutralising point and direct to the neutral.

I think the original question goes back to the common enough lack of understanding that people won't get shocked if their body isn't in contact with earth.

I think the bigger lack of understanding is, that most assume contact with a 230v source means an automatic shock when standing on the ground.

Merch

robbie7730 wrote: »

Yea I guess he is actually saying a fault. But the fault that causes such a scenario as mentioned is a neutral failure before or right at the meter cutout.

Failed neutral = broken neutral or open neutral. Usually caused by failure of a connection.

And in a situation with an open neutral (for whatever reason) the electrical supply would still be functional?

Armin_Tamzarian wrote: »

That's the point of earthing, the excessive current flowing to earth causes the overcurrent device to trip, i.e. the breaker/fuse.
If there was no decent earth path the level of fault current might remain too low to trip the breaker or blow the fuse.
Even if the device in question doesn't have it's own fuse, the breaker feeding that circuit will trip.

I think the original question goes back to the common enough lack of understanding that people won't get shocked if their body isn't in contact with earth.

The bolded parts are the bit i thought were the case, it was described to me that the current would go infinitely high momentarily causing the main breaker or fuse to trip/blow. but as you say without a decent earth (or ineffective earth) that may not occur.So a device (fused or not) still may be live? as its own circuit may also not trip for the same reason? not high enough current or for long enough?

Armin_Tamzarian

robbie7730 wrote: »

The main point of equipotential bonding is to ensure all metal remains at equal potential during fault conditions. The most likely fault it protects against in neutralised systems is a failed neutral.

I talked about earthing, you're talking about bonding.

robbie7730 wrote: »

I think the bigger lack of understanding is, that most assume contact with a 230v source means an automatic shock when standing on the ground.

Well, lots don't understand that you need to be completing a circuit to earth or no shock will occur.
But further to that, many people who would understand that would not know that standing on a ground floor level floor would usually not provide a good path to earth.

Merch

Armin_Tamzarian wrote: »

I think the original question goes back to the common enough lack of understanding that people won't get shocked if their body isn't in contact with earth.

eh? but if current follows the path of least resistance, and you are isolated from anywhere for it to go by being "not in contact with earth" even if that isolation is a pair of rubber soled shoes, surely that adds protection from shock as there is no path?
There are specific rubber mats that are used when working on electrical installations to prevent/limit this?

I mean this in that, i assume a person is not in contact with ground or any other conductor that is earthed somewhere to ground.

Are you sure you meant isn't as in is not in contact,
did you mean is?

Am I readin that correct or is it double negatives?

is this the same thing as what you are saying (what Robbie7730 said)

"I think the bigger lack of understanding is, that most assume contact with a 230v source means an automatic shock when standing on the ground."

Sorry, i think I read that wrong, thought you were saying people will definitely get a shock if they arent grounded but that people generally misunderstand that.

I see you are saying people wont get a shock if they are not grounded. (as in they could be hanging out of a live wire and nothing will occur until they are grounded) (voltage dependant)

Armin_Tamzarian

Merch wrote: »

And in a situation with an open neutral (for whatever reason) the electrical supply would still be functional?

No, but though certain things could act as a neutral and allow a circuit to function, to a degree.
If it was say TN-CS, the neutral/earth connection could cause the earth to sort of function as the neutral.

Merch wrote: »

The bolded parts are the bit i thought were the case, it was described to me that the current would go infinitely high momentarily causing the main breaker or fuse to trip/blow. but as you say without a decent earth (or ineffective earth) that may not occur.So a device (fused or not) still may be live? as its own circuit may also not trip for the same reason? not high enough current or for long enough?

Yes, X amount of current is required to trip a breaker.
Without a decent return path the current flow may be low enough to trip the breaker.
The better the connection the lower the impedance.
The lower the impedance the higher the current.
Hence the reason testing requires lower impedances the greater the size of the protective device.
The current won't be infinitely high.
You can work out the maximum fault current based on the earth loop impedance readings.

Bruthal

Merch wrote: »

And in a situation with an open neutral (for whatever reason) the electrical supply would still be functional?

If the failed neutral is inside the meter, the installation goes completely dead, and there is no rise of the metalwork voltage as the neutral is still intact at the neutralising point.

If the failure is outside the meter, the earth rod now connects the broken neutral from the house to the ground, but it will not be good enough for the house to function, and the voltage on metal work will rise. But once all metal is bonded, it will all be at the same potential, to reduce shock risk.

The bolded parts are the bit i thought were the case, it was described to me that the current would go infinitely high momentarily causing the main breaker or fuse to trip/blow. but as you say without a decent earth (or ineffective earth) that may not occur.So a device (fused or not) still may be live? as its own circuit may also not trip for the same reason? not high enough current or for long enough?

It wont go infinitely high, it will go to the level of voltage/impedance, which should be high enough to trip devices. This is facilitated by the neutralising connection which connects the earthing system direct to the neutral, to keep the fault path impedance low, and therefore the fault current high to quickly operate protective devices.

On non neutralised systems, the impedance can be relatively high, and so the installation would be covered by a main RCD with a 200 or 300ma sensitivity i think it is, usually with a slight time delay to allow discrimination with final circuit RCD`s.

Bruthal

Armin_Tamzarian wrote: »

I talked about earthing, you're talking about bonding.
.

The original post asked about bonding.

Armin_Tamzarian

Merch wrote: »

eh? but if current follows the path of least resistance, and you are isolated from anywhere for it to go by being "not in contact with earth" even if that isolation is a pair of rubber soled shoes, surely that adds protection from shock as there is no path?
There are specific rubber mats that are used when working on electrical installations to prevent/limit this?

I mean this in that, i assume a person is not in contact with ground or any other conductor that is earthed somewhere to ground.

Are you sure you meant isn't as in is not in contact,
did you mean is?

Am I readin that correct or is it double negatives?

is this the same thing or what you are saying (what Robbie7730 said)

"I think the bigger lack of understanding is, that most assume contact with a 230v source means an automatic shock when standing on the ground."

Sorry, i think I read that wrong, thought you were saying people will definitely get a shock if in
people they arent grounded but that people generally misunderstand that.

I see you are saying people wont get a shock if they are not grounded.

No the double negative is correct.
They won't get shocked if they aren't in contact with earth.
As in, they will get shocked if they are in contact with earth.
Point being that a common misconception would be that you get shocked every time you touch a live wire.
That you need to be creating an earth path for the shock to occur.

Armin_Tamzarian

robbie7730 wrote: »

The original post asked about bonding.

It certainly does but if you look back at my post you will see that I was responding to Merch, who asked a question about earthing.
Sorry for taking your thread off track, I was just offering an answer to the question.

Merch

robbie7730 wrote: »

If the failed neutral is inside the meter, the installation goes completely dead, and there is no rise of the metalwork voltage as the neutral is still intact at the neutralising point.

If the failure is outside the meter, the earth rod now connects the broken neutral from the house to the ground, but it will not be good enough for the house to function, and the voltage on metal work will rise. But once all metal is bonded, it will all be at the same potential, to reduce shock risk.

To expand on that, and I know this is outside the OP and maybe probability,

If the Neutral was damaged and the earth was either damaged or the bonding was not good/functional between all bonded components , this seems to present a worst case, in that a person may then earth a live conductor?

Sorry to drag this off topic

Armin_Tamzarian

Merch wrote: »

To expand on that, and I know this is outside the OP and maybe probability,

If the Neutral was damaged and the earth was either damaged or the bonding was not good/functional between all bonded components , this seems to present a worst case, in that a person may then earth a live conductor?

Yes, if they were forming a path to earth through their body.
Typically at 230V it's not enough to kill, at that voltage not much current is required to kill you if it flows through you, less than an amp.
However, our body's impedance is fairly high and our connection to earth is pretty low so we're usually alright.
That's the reason RCDs are required as additional protection in certain locations.
If we are wet, the impedance to earth can be lowered.

Bruthal

Merch wrote: »

To expand on that, and I know this is outside the OP and maybe probability,

If the Neutral was damaged and the earth was either damaged or the bonding was not good/functional between all bonded components , this seems to present a worst case, in that a person may then earth a live conductor?

If the earth rod itself was disconnected from the db, it wouldnt make a major difference once the bonding was good. But how much difference depends on how good the earth rod connection to the earth was, and how much load was on, which is explained in the last paragraph below.

If the bonding was not complete between different pipes etc, and the pipes were not connected physically to each other, that could present a danger, as the non bonded one might remain down at earth potential, with the bonded ones at or up toward 230v.

The voltage level on the bonded metal depends on the load on in the house, and the earth rod path impedance. The load impedance and earth rod to ground impedance are like 2 resistors in series, with theneutralised connection to the bnded metal connected in the middle. So the higher the load impedance (less items switched on) compared to the earth rod impedance, the lower the touch voltage on the bonded metal.

The above is in a failed neutral scenario in a tn-cs setup.

Merch

Armin_Tamzarian wrote: »

Yes, if they were forming a path to earth through their body.
Typically at 230V it's not enough to kill, at that voltage not much current is required to kill you if it flows through you, less than an amp.
However, our body's impedance is fairly high and our connection to earth is pretty low so we're usually alright.
That's the reason RCDs are required as additional protection in certain locations.
If we are wet, the impedance to earth can be lowered.

I believe it varies from person to person, I read between 300-800mA? all the same, I dont want to test that.
It does make me wonder why rcd's aren't used in all circuits of some domestic installations (as far as I have seen, as in an existing set up, eg my own) cooker isnt on an RCD, immersions?

Bruthal

Armin_Tamzarian wrote: »

Typically at 230V it's not enough to kill, at that voltage not much current is required to kill .

Thats another slight misconception imo. The current is the current.

If 230v puts 50 ma through a person and it kills them, 170v with a better connection can also put 50ma through, and the shock would be identical.

There is an external resistance which varys widely. A person is like 3 resistors in series for 2 points of contact. The internal resistance is stable. The external resistances vary widely depending on the quality of contact, wet hands, surface area of contact etc.

So the 50ma as an example current will have a certain voltage drop across the persons internal resistance. If its 230v that causes this 50ma, or if its 170v that causes it, the difference is in the external resistance.

Bruthal

Merch wrote: »

I believe it varies from person to person, I read between 300-800mA? all the same, I dont want to test that.
It does make me wonder why rcd's aren't used in all circuits of some domestic installations (as far as I have seen, as in an existing set up, eg my own) cooker isnt on an RCD, immersions?

800ma would be rapid death for anything but milli seconds duration.

Merch

robbie7730 wrote: »

This is facilitated by the neutralising connection which connects the earthing system direct to the neutral, to keep the fault path impedance low, and therefore the fault current high to quickly operate protective devices.

On non neutralised systems, the impedance can be relatively high, and so the installation would be covered by a main RCD with a 200 or 300ma sensitivity i think it is, usually with a slight time delay to allow discrimination with final circuit RCD`s.

This is what connects the neutral to the earth? at the transformer?
If so, I wondered how they were connected, knew it was somewhere at a transformer

robbie7730 wrote: »

Thats another slight misconception imo. The current is the current.

If 230v puts 50 ma through a person and it kills them, 170v with a better connection can also put 50ma through, and the shock would be identical.

There is an external resistance which varys widely. A person is like 3 resistors in series for 2 points of contact. The internal resistance is stable. The external resistances vary widely depending on the quality of contact, wet hands, surface area of contact etc.

So the 50ma as an example current will have a certain voltage drop across the persons internal resistance. If its 230v that causes this 50ma, or if its 170v that causes it, the difference is in the external resistance.

Surface area of contact? so a greater surface area is worse or better for increasing reistance? if its a lesser surface area that causes less resistance, how do you let go? ok, not that funny.
I've read cuts in the skin can do that too (reduce resistance) i assumed the reason one person does and another doesnt get shocked is the variation in their own body's resitance, Im trying to think how less or more voltage affects peopleof differing resistances.

Armin_Tamzarian

robbie7730 wrote: »

If the earth rod itself was disconnected from the db, it wouldnt make a major difference once the bonding was good. But how much difference depends on how good the earth rod connection to the earth was, and how much load was on, which is explained in the last paragraph below.

But, if it was TT, which it would be with a rod, if the rod is disconnected then what would be providing the path to earth?
You could talk about extraneous sources but those haven't been mentioned.
If the person was already connected to earth then the bonding would be irrelevant and touching any exposed metalwork would shock them.
But if the earth rod had been connected the protective device would have tripped.
So, to answer Merch's question, I'm not sure if you'd call it a "worst case", but it would certainly be worse because the effectiveness of the protective device would have been reduced.

robbie7730 wrote: »

The voltage level on the bonded metal depends on the load on in the house, and the earth rod path impedance. The load impedance and earth rod to ground impedance are like 2 resistors in series, with theneutralised connection to the bnded metal connected in the middle. So the higher the load impedance (less items switched on) compared to the earth rod impedance, the lower the touch voltage on the bonded metal.

I'm not entirely sure what you mean with what you say about the effect of the load impedance.
If a live conductor carrying 230v connects with something metallic then the metallic object will also have a potential of 230v.
If the object's connection to earth is 0 then it will have a potential of 0v.
I don't see how load impedance would have any effect on that?
I'm genuinely interested, it might just be something I didn't know about, it happens.

Bruthal

Merch wrote: »

This is what connects the neutral to the earth? at the transformer?
If so, I wondered how they were connected, knew it was somewhere at a transformer

Earthing would happen at trandformer neutrals whether the houses are neutraised or not. This is why the earth as a whole is a return path, because the general mass of earth is connected to the transformer neutral.

The house neutralising connection is done at the meter cutout fuse unit.

Surface area of contact? so a greater surface area is worse or better for increasing reistance? if its a lesser surface area that causes less resistance, how do you let go? ok, not that funny.
I've read cuts in the skin can do that too (reduce resistance) i assumed the reason one person does and another doesnt get shocked is the variation in their own body's resitance, Im trying to think how less or more voltage affects peopleof differing resistances.

The greater the surface area of contact, the worse the shock.

Different people will get shocked similarly. If one does and another does not, it will be because one had good contact in at least 2 places, while the other did not.

Bruthal

Armin_Tamzarian wrote: »

But, if it was TT, which it would be with a rod, if the rod is disconnected then what would be providing the path to earth?
You could talk about extraneous sources but those haven't been mentioned.
If the person was already connected to earth then the bonding would be irrelevant and touching any exposed metalwork would shock them.
But if the earth rod had been connected the protective device would have tripped.
So, to answer Merch's question, I'm not sure if you'd call it a "worst case", but it would certainly be worse because the effectiveness of the protective device would have been reduced.

Earth rods are not exclusive to TT systems.

I'm not entirely sure what you mean with what you say about the effect of the load impedance.
If a live conductor carrying 230v connects with something metallic then the metallic object will also have a potential of 230v.
If the object's connection to earth is 0 then it will have a potential of 0v.
I don't see how load impedance would have any effect on that?
I'm genuinely interested, it might just be something I didn't know about, it happens.

Both of my posts above are with reference to neutralised installations with failed neutrals. I can draw out an illustration of the load impedance in series with the earth rod to determine touch voltage if you like, but since I mention it was with reference to a failed neutral in a tn-cs setup, you probably know what I was getting at now.

Armin_Tamzarian

Merch wrote: »

I believe it varies from person to person, I read between 300-800mA? all the same, I dont want to test that.
It does make me wonder why rcd's aren't used in all circuits of some domestic installations (as far as I have seen, as in an existing set up, eg my own) cooker isnt on an RCD, immersions?

I think the figure is lower than that but yeah, it varies per person.
Fatter people have a lower water content in their bodies and therefore have greater impedance and as a result they are less likely to die from a shock.
I think that's how those electronic body fat testers work, by working out your body fat based on your impedance.

robbie7730 wrote: »

Thats another slight misconception imo. The current is the current.

I never said that it wasn't.
I only said that at 230v that not much current is required to kill, it's not.

Merch wrote: »

This is what connects the neutral to the earth? at the transformer?
If so, I wondered how they were connected, knew it was somewhere at a transformer



Surface area of contact? so a greater surface area is worse or better for increasing reistance? if its a lesser surface area that causes less resistance, how do you let go? ok, not that funny.
I've read cuts in the skin can do that too (reduce resistance) i assumed the reason one person does and another doesnt get shocked is the variation in their own body's resitance, Im trying to think how less or more voltage affects peopleof differing resistances.

Yes, the same way that the larger the cable, the lower the impedance.
Depending on both cables' metals being the same of course robbie
The greater the connection the more current that can flow as the impedance is being lowered.

Armin_Tamzarian

robbie7730 wrote: »

Earth rods are not exclusive to TT systems.

Both of my posts above are with reference to neutralised installations with failed neutrals. I can draw out an illustration of the load impedance in series with the earth rod to determine touch voltage if you like, but since I mention it was with reference to a failed neutral in a tn-cs setup, you probably know what I was getting at now.

Ok, it's just that you started your post with "If the earth rod itself was disconnected from the db", so you can see why I wrongfully presumed that you were referring to a TT installation.
Typically if rods are used in a non-TT setup they are more of a backup than anything else and their being disconnected would typically not great any increased danger.

Bruthal

Armin_Tamzarian wrote: »

I never said that it wasn't.
I only said that at 230v that not much current is required to kill, it's not.
.

But no more current would be required to kill at 12v or 50v.

But them voltages wont drive enough current to kill.

If 50ma was the kill level, it wont make any difference what the supply voltage was that drove the 50ma current.
So saying that at 230v, not much current is needed, the 230v part is irrelevant really, except as a reference to give an idea of the magnitude of shock a person has actually received.

Bruthal

Armin_Tamzarian wrote: »

Ok, it's just that you started your post with "If the earth rod itself was disconnected from the db", so you can see why I wrongfully presumed that you were referring to a TT installation.
Typically if rods are used in a non-TT setup they are more of a backup than anything else and their being disconnected would typically not great any increased danger.

The way I always seen an earth rod on a tn-cs estate was, that it is another one added to the collective. More earth rods on the neutral of the estate, all connected together.