Neutral current on a metal plumbing piping system

[Bro8898]

Hey guys. I’m a plumber and have posted here before. Again, I’ve got a question about neutral current on a metal plumbing piping system. It was explained to me very well in a previous thread how this can occur.

Could anyone inform me on how I will know if there’s neutral current on a metal piping system? Why won’t I get shocked if I just touch the pipe but it’s only when I open the pipe up and have one hand touching each end then I can be shocked? I would like to know ahead of time (if possible) how I can tell if there’s neutral current on the pipe before opening it up to make a repair.

Thanks in advance for any help and feedback. You’ve all been very helpful in the past and I appreciate it very much.

 

[jaggedben]

You won't get shocked unless you touch the pipe and something else that has a voltage to the pipe at the same time. However if the pipe is carrying most or all of the current from some electrical circuit, you'll get an electrical arc if you make or break that connection. Also while the pipe is intact, even if there's current flowing on it, one part of the pipe cannot have a voltage to another part within reach. However, once you take it apart, the two separated pieces could now have a voltage to each other that can shock you if your touching both pieces or arc when reconnected. A dangerous voltage difference can only really between two disconnected metal parts (at least for your purposes), or between an ungrounded part and the earth.

Hope that helps with the theory.

I can't think of a simple-to-explain easy way to be absolutely sure a pipe is safe to cut but here are some high percentage plays.

> Get an amp clamp and put it around a pipe before you cut the pipe. If there's current flowing on the pipe, that's a red flag, get an electrician to check it out before cutting. This works for pipes on the house past the point where the electrical is grounded to the water pipe.

> For a metal water service pipe from the street, a small amount of current (a couple amps) may be normal and okay. But compared to the neutral wire from the electrical service it should be low. Find a place to amp clamp the electrical neutral, say, at the weatherhead on a aone story house, or in the service panel if you can do it safely. If there is no current on the electrical neutral and current on the water service pipe, that's a red flag, get an electrician to fix the electrical neutral before cutting the pipe. This situation involves not only hazard to you but also the customers electrical stuff.

Again, this is not exhaustive. It's hard to be exhaustive. But it can help.

 

[synchro]

In addition to the measurements jaggedben mentioned, if you are concerned you could place a ground clamp on either side of the region where you are doing cutting, removing pipe, etc. Then connect the clamps together with a wire that's long enough to get it out of your way while you're doing the work.

 

[tom baker]

 

[Bro8898]

You won't get shocked unless you touch the pipe and something else that has a voltage to the pipe at the same time. However if the pipe is carrying most or all of the current from some electrical circuit, you'll get an electrical arc if you make or break that connection. Also while the pipe is intact, even if there's current flowing on it, one part of the pipe cannot have a voltage to another part within reach. However, once you take it apart, the two separated pieces could now have a voltage to each other that can shock you if your touching both pieces or arc when reconnected. A dangerous voltage difference can only really between two disconnected metal parts (at least for your purposes), or between an ungrounded part and the earth.

Hope that helps with the theory.

I can't think of a simple-to-explain easy way to be absolutely sure a pipe is safe to cut but here are some high percentage plays.

> Get an amp clamp and put it around a pipe before you cut the pipe. If there's current flowing on the pipe, that's a red flag, get an electrician to check it out before cutting. This works for pipes on the house past the point where the electrical is grounded to the water pipe.

> For a metal water service pipe from the street, a small amount of current (a couple amps) may be normal and okay. But compared to the neutral wire from the electrical service it should be low. Find a place to amp clamp the electrical neutral, say, at the weatherhead on a aone story house, or in the service panel if you can do it safely. If there is no current on the electrical neutral and current on the water service pipe, that's a red flag, get an electrician to fix the electrical neutral before cutting the pipe. This situation involves not only hazard to you but also the customers electrical stuff.

Again, this is not exhaustive. It's hard to be exhaustive. But it can help.

Very interesting. Thank you for that reply. Not saying that I would do this but if I took the cover off the main panel and measured from line to neutral on both of the incoming lines would I see strange voltages between the two? Meaning not 120 and 120. Thanks

 

[Bro8898]

In addition to the measurements jaggedben mentioned, if you are concerned you could place a ground clamp on either side of the region where you are doing cutting, removing pipe, etc. Then connect the clamps together with a wire that's long enough to get it out of your way while you're doing the work.

I actually made one of those just for that purpose. It’s made of two pipe clamps with a 4 gauge stranded wire. Do you think 4 gauge is big enough for this purpose? It would only be used in residential applications. Thanks

 

[Bro8898]

Very interesting video. I watched it and am learning. Thanks for sending

 

[zbang]

Do you think 4 gauge is big enough for this purpose?

For residential, yes, assuming it's not a Huge Mansion. Main thing is that if you apply the jumper and cut the pipe, the current will now be running in that jumper and can be metered with the clamp-on if you want.

(A clamp-on meter for this sort of thing doesn't need to cost $300, a cheap one will do the trick.)

 

[LarryFine]

You can use a set of automotive jumper cables; split them to make two jumpers.

 

[winnie]

Voltage is not a measurement of a single thing, it is always a measurement between two things. To get shocked, you need to touch two things with a large enough voltage between them.

This is why birds can stand on electrical wires without harm. They are touching one thing. If a bird manages to touch both an electrical wire and a nearby grounded item, that bird has a bad day.

One important aspect of electrical safety is bonding, where metal objects which might have a voltage between them are connected together using a conductor. Any voltage difference will cause current to flow, and this current tends to 'use up' whatever is creating the voltage difference.

If the cause of the voltage difference is something low current like static charging, bonding can completely eliminate it. This is seen in plumbing for insulating fluids such a fuel, where bonding is used to prevent sparks to prevent fires.

If the cause of the voltage difference is a high current source, then the bonding will hopefully allow enough current to flow to trip a breaker.

In the case of things such metal water lines with common underground metal piping, you can have a situation where the pipes act as a parallel neutral. Some of the current which is supposed to flow on the neutral wire flows on the pipes.

The neutral is a 'grounded conductor', meaning that even with current flowing it is maintained at near soil voltage. The voltage between this neutral and soil is low, so if you touch both it doesn't shock you.

In a normal functioning system, where the pipe is parallel to the neutral, if you cut the pipe you still have the neutral present. The neutral is a conductor, and it is connected (via a circuitous and unintentional path) to both ends of the cut, bonding them together. In such a system you might measure current on the pipe, but when you cut the pipe there isn't a shock hazard because of the bonding.

The danger comes when the parallel pipe hides a broken neutral . In this case all of the neutral current flows on the pipe, and when you cut the pipe there is no bonding to keep the voltage low between the two ends. In this case when you cut the pipe you end up with a shock hazard and possible electrical damage throughout the house.

The bonding jumper is a great way to mitigate this risk.

Jon

 

[Dennis Alwon]

Hey guys. I’m a plumber and have posted here before. Again, I’ve got a question about neutral current on a metal plumbing piping system. It was explained to me very well in a previous thread how this can occur.

Could anyone inform me on how I will know if there’s neutral current on a metal piping system? Why won’t I get shocked if I just touch the pipe but it’s only when I open the pipe up and have one hand touching each end then I can be shocked? I would like to know ahead of time (if possible) how I can tell if there’s neutral current on the pipe before opening it up to make a repair.

Thanks in advance for any help and feedback. You’ve all been very helpful in the past and I appreciate it very much.

If you are touching a pipe with voltage and you are kneeling in a crawl space, and just for kicks you have a hole in the knees, then you could possibly get a shock.

 

[ptonsparky]

A plumber jumped in a hole they dug to cut the metal water line using a sawzall. Back then I was surprised it had an intact EG all the way back to a neighboring building. There was enough current flow that it melted the teeth of the blade. I was told it did not take long for the plumber to exit the hole. Funny now.

Amp clamp the pipe.

 

[jaggedben]

Very interesting. Thank you for that reply. Not saying that I would do this but if I took the cover off the main panel and measured from line to neutral on both of the incoming lines would I see strange voltages between the two? Meaning not 120 and 120. Thanks

First of all 120/240 is nominal voltage for most single phase residential. Meaning the voltage could be a bit higher or lower - I've routinely seen anywhere from about 115 to 127 myself - and that isn't strange.

What's more of a red flag is unbalanced voltages, i.e. they don't match within a volt or two, or one or both is high or low more than 10-12 volts different from 120. That's a sign of open or weak neutral which means more current could be flowing on the ground wire to a metal water service.

(I'm not gonna go over three phase here.)

 

[Bro8898]

That’s good info. Thanks for the response. No need to go over three phase. Thank you

 

[Bro8898]

Voltage is not a measurement of a single thing, it is always a measurement between two things. To get shocked, you need to touch two things with a large enough voltage between them.

This is why birds can stand on electrical wires without harm. They are touching one thing. If a bird manages to touch both an electrical wire and a nearby grounded item, that bird has a bad day.

One important aspect of electrical safety is bonding, where metal objects which might have a voltage between them are connected together using a conductor. Any voltage difference will cause current to flow, and this current tends to 'use up' whatever is creating the voltage difference.

If the cause of the voltage difference is something low current like static charging, bonding can completely eliminate it. This is seen in plumbing for insulating fluids such a fuel, where bonding is used to prevent sparks to prevent fires.

If the cause of the voltage difference is a high current source, then the bonding will hopefully allow enough current to flow to trip a breaker.

In the case of things such metal water lines with common underground metal piping, you can have a situation where the pipes act as a parallel neutral. Some of the current which is supposed to flow on the neutral wire flows on the pipes.

The neutral is a 'grounded conductor', meaning that even with current flowing it is maintained at near soil voltage. The voltage between this neutral and soil is low, so if you touch both it doesn't shock you.

In a normal functioning system, where the pipe is parallel to the neutral, if you cut the pipe you still have the neutral present. The neutral is a conductor, and it is connected (via a circuitous and unintentional path) to both ends of the cut, bonding them together. In such a system you might measure current on the pipe, but when you cut the pipe there isn't a shock hazard because of the bonding.

The danger comes when the parallel pipe hides a broken neutral . In this case all of the neutral current flows on the pipe, and when you cut the pipe there is no bonding to keep the voltage low between the two ends. In this case when you cut the pipe you end up with a shock hazard and possible electrical damage throughout the house.

The bonding jumper is a great way to mitigate this risk.

Jon

Thank you for that reply. That’s the part I’m trying to understand and probably need to study more in that area. Just to confirm, when I use the bonding jumper it only needs to be used when opening up a pipe on the “street” side of the bonding jumper right? I don’t need to use it within the house, does that sound right to you? Thanks again for the informative response.

 

[ptonsparky]

Thank you for that reply. That’s the part I’m trying to understand and probably need to study more in that area. Just to confirm, when I use the bonding jumper it only needs to be used when opening up a pipe on the “street” side of the bonding jumper right? I don’t need to use it within the house, does that sound right to you? Thanks again for the informative response.

Kind of depends on where the bond is made. At one time we just needed to bond the water line. Typically we hit the closest cold water line. That didn't mean it was even close to where it is entered the house.

 

[jaggedben]

Thank you for that reply. That’s the part I’m trying to understand and probably need to study more in that area. Just to confirm, when I use the bonding jumper it only needs to be used when opening up a pipe on the “street” side of the bonding jumper right? I don’t need to use it within the house, does that sound right to you? Thanks again for the informative response.

If you are dealing with a broken service neutral then yes, this is an issue for the pipe going to the street, because it may lead to the neighbors house which allows the current to go back to the utility on the neighbors neutral.

That is not to say 100% that there couldn't be other situations where cutting a pipe inside the house could lead to shock or arc flash, and where the bonding jumper could stop or mitigate that. Those are still possible in a fluky or sketchy situation. Say, if someone deliberately used the pipe as neutral return, i.e. some awful DIY hack job that was completely unsafe but 'worked' at the outlet. Or some crazy fluke involving a combination of multiple errors and/or accident. So possible, just less likely, and maybe you can judge the likelihood in part by the age and condition of the house.

 

[Jpflex]

Hey guys. I’m a plumber and have posted here before. Again, I’ve got a question about neutral current on a metal plumbing piping system. It was explained to me very well in a previous thread how this can occur.

Could anyone inform me on how I will know if there’s neutral current on a metal piping system? Why won’t I get shocked if I just touch the pipe but it’s only when I open the pipe up and have one hand touching each end then I can be shocked? I would like to know ahead of time (if possible) how I can tell if there’s neutral current on the pipe before opening it up to make a repair.

Thanks in advance for any help and feedback. You’ve all been very helpful in the past and I appreciate it very much.

As far as I know, this situation is called objectionable current. One of the easiest ways this can happen is if a common receptacle has its ground pig tail accidentally touching left side neutral conductors of receptacle. It happens because wires are shoved in a small receptacle box. Wire taping around receptacle box connections helps mitigate this.

A neutral touching ground will place current on everything bonded or connected to ground, including the metal pipes you repair.

It only takes a few mili amperes (Fraction of an amp) to kill someone. No one has touched on what values he should expect when placing an amp clamp around pipes. Does anyone have suggestions here?

The person who suggested placing a jumper between the pipes you are to cut is a great idea since this will bond the two pieces even when cut in order to keep them at the same potential or no voltage between them. This makes it much safer for you.

 

[Jpflex]

Also while wearing gloves and removing jumper wire, I was thinking that depending on level of volts or current you may be able to place a test light as you did jumper such as automotive test light between cut pipes and if there is any hint of illumination, you had current on it.

 

[ActionDave]

As far as I know, this situation is called objectionable current. One of the easiest ways this can happen is if a common receptacle has its ground pig tail accidentally touching left side neutral conductors of receptacle. It happens because wires are shoved in a small receptacle box. Wire taping around receptacle box connections helps mitigate this. ....

Your example of a neutral touching a metal box is an example of objectionable current. Current on a metal underground water pipe is a byproduct of a properly bonded electrical system.