Objectionable current

[CAPTMIKE]

The NEC has specific criteria for installing an electrical system to prevent objectionable current, and also gives ways to stop it, 250.6 (A)(B). On existing installations this may be a not so easy problem to find although it can be corrected, using proper methods.
At what point is (OC) considered dangerous, I am having difficulty finding specific data to define a point at which it becomes hazardous. The NEC has no parameters and the way I see it is there is NO safe allowable (OC). Any help would be appreciated.

 

[ptonsparky]

It will depend on your installation. 5 amps of current flowing on a neighborhood waterpiping system may just be normal because of all the neutrals bonded to the water pipe. The same current for a single residence with its own water supply would indicate trouble.

Leakage current on old CRT monitors were high enough that they could trip a GFCI.

 

[brian john]

Commercial, residential, SDS or main service, how big a service, types of loads?

Lots of variables.

 

[Pierre C Belarge]

I believe that current flowing outside of the "normal" path is/should be considered a potential hazard.

As you mentioned, sometimes finding the cause can be very tedious/time consuming.

 

[brian john]

We trace down ground current all the time. We utilize a 4 step approach.

1st a visual inspection and try to couple this with an IR only because it is practical as panel covers have to be pulled. WE locate a slew of grounded neutrals during this portion of the inspection.

2nd as part of the visual we take voltage readings neutral to ground at each distribution point.

3rd as part of the visual we take zero sequence readings at each distribution point where it is safe and practical.

4th during a scheduled outage we repair any items noted during the visual and then isolate the feeders and megger them all conductors, if all feeders are clear we next megger all branch circuit neutrals and megger them. WEE tag all conductors noted as grounded and either repair them as part of the inspection, if time allows or return and repair at a later date.

 

[Cold Fusion]

---At what point is (OC) considered dangerous, I am having difficulty finding specific data to define a point at which it becomes hazardous. The NEC has no parameters and the way I see it is there is NO safe allowable (OC). Any help would be appreciated.

This is an area of confusion for me as well. In one case I can think of, the NEC actually demands an arrangement that will give significant grounding conductor current - baffles me completely.

My inclination is there shiould not be any current on the grounding conductors. If for no other reason than to reduce the galvanic corrosion of underground piping

But I don't know why it is dangerous. And I don't know why it is not dangerous on the utility side of the service disconnect.

Perhaps our experts could start with why ground current is dangerous and why that changes at the service disconnect.

cf

 

[erickench]

Objectionable current causes the following hazards:

1) Shock Hazard - neutral current flowing on metal parts can cause injury/death.

2) Fire Hazard - neutral current flowing on metal parts can ignite adjacent combustible material.

3) Improper Operation of Electronic Equipment - neutral current flowing on metal parts can cause electromagnetic fields which negatively affect the performance of electronic devices.

4) Operation of Overcurrent Devices - when neutral current travels on metal parts, tripping of electronic overcurrent devices equipped with ground-fault protection can occur because neutral current is flowing on the EGC.

 

[don_resqcapt19]

Objectionable current causes the following hazards:

1) Shock Hazard - neutral current flowing on metal parts can cause injury/death.

2) Fire Hazard - neutral current flowing on metal parts can ignite adjacent combustible material.

3) Improper Operation of Electronic Equipment - neutral current flowing on metal parts can cause electromagnetic fields which negatively affect the performance of electronic devices.

4) Operation of Overcurrent Devices - when neutral current travels on metal parts, tripping of electronic overcurrent devices equipped with ground-fault protection can occur because neutral current is flowing on the EGC.

Eric,
I agree with all of that, but the NEC not only permits parallel paths for neutral current on the line side of the disconnect, it actually requires them.

 

[George Stolz]

One observation when discussing objectionable current is that in the 2008 revision cycle a proposal was submitted to require neutrals at wall switch locations, because some dimmers were designed to use the equipment grounding conductor as a stand-in for a neutral. If I recall correctly, a majority of CMP-5 took issue with the idea of a device designed to release even milliamps of objectionable current into the equipment grounding system.

 

[wirenut1980]

Whether or not objectionable current on the grounding system is dangerous is determined on a case by case basis. I have investigated cases where there are over 5 amps flowing on the grounding system. The cause is due to NEC and NESC bonding of the grounding systems of power and communications on both the customer side and utility side of the service.

Most of the time, the high resistance of the human body makes the current not harmful to people, since there is not enough voltage to push the current through the human body's resistance.

 

[brian john]

Whether or not objectionable current on the grounding system is dangerous is determined on a case by case basis. I have investigated cases where there are over 5 amps flowing on the grounding system. The cause is due to NEC and NESC bonding of the grounding systems of power and communications on both the customer side and utility side of the service.

Most of the time, the high resistance of the human body makes the current not harmful to people, since there is not enough voltage to push the current through the human body's resistance.

We had a job in Baltimore with 240-400 amps of ground cirrent (varied with load). Found a cracked insulator between the neutral bus and ground bus in a 4000 amp busway.

 

[erickench]

Eric,
I agree with all of that, but the NEC not only permits parallel paths for neutral current on the line side of the disconnect, it actually requires them.

By line side I assume you mean the supply side of the disconnect. I don't think utilities use a grounding conductor. The neutral is the fault current return path. In fact I don't even think they use metal raceway for their service feeders. Am I wrong?

 

[iwire]

By line side I assume you mean the supply side of the disconnect. I don't think utilities use a grounding conductor. The neutral is the fault current return path. In fact I don't even think they use metal raceway for their service feeders.

To be clear the line side of the service disconnect. Often the NEC applies to the work between the "Service Point" and the service disconnect.


Picture a typical single family home overhead service. The installer chooses to use RMC from weatherhead to panel. So RMC down from the weather head into a meter socket mounted outside the home, then more RMC into the building and into the service disconnect enclosure.

The NEC requires the meter socket and the service disconnect enclosure to be bonded to the neutral. The RMC between the meter socket and the service disconnect is now in parallel with the neutral conductor. This is very common in many installations.

 

[erickench]

Yes but if there's a fault on that RMC between the weatherhead and the meter enclosure then the breaker will still trip. The current will flow through the main bonding jumper at the disconnect. There is no continuous metal raceway from the weatherhead to the utility transformer. Therefore current does not flow directly back to the utility through any metal raceway. It must first make a detour through the disconnect.

 

[erickench]

The NEC requires only one main bonding jumper and that is in the enclosure. Any fault occurring in the meter enclosure and the service conduit would result in all the current flowing through this jumper.

 

[don_resqcapt19]

Yes but if there's a fault on that RMC between the weatherhead and the meter enclosure then the breaker will still trip. The current will flow through the main bonding jumper at the disconnect. There is no continuous metal raceway from the weatherhead to the utility transformer. Therefore current does not flow directly back to the utility. It must first make a detour through the disconnect.

The current is on the line side of the breaker and the breaker will not see that current or trip. The path is from the fault through the conduit to the main bonding jumper, to the grounded conductor and back to the transformer. None of this current will go through the breaker.

As far as the parallel paths that are required I was not thinking about the service equipment itself. One path is the TV cable system. The shield is bonded to the building grounding system at every building and provides a parallel path for the grounded conductor current. Underground telephone cables will do the same, and if you have a common metal underground water pipe system in the area, it would not be uncommon to find 20% or more of the grounded conductor current returning to the transformer via the water pipes.

 

[iwire]

The NEC requires only one main bonding jumper and that is in the enclosure.

I agree the NEC requires only one main bonding jumper and the location of that jumper is required to be in the enclosure containing the service disconnect.

But now back outside is a meter enclosure that needs to be bonded and it will be bonded to the neutral. It is not a main bonding jumper but it does bond the meter enclosure to the neutral.

 

[erickench]

I believe that communications cable shield is just there to guard against electromagnetic interference. As for the water pipe it's obviously being used as a grounding electrode and the utility transformer could be grounded as well. You know what would be a good idea? If they connected a lightning arrestor in the grounding electrode circuit this would eliminate any stray earth currents and prevent electrocution. And if lightning strikes the system it would discharge though the arrestor. What do you think?

 

[erickench]

But now back outside is a meter enclosure that needs to be bonded and it will be bonded to the neutral. It is not a main bonding jumper but it does bond the meter enclosure to the neutral.

How is this meter enclosure bonded to the neutral? It should'nt be. I don't even think the grounding electrode should be connected to the meter enclosure.

 

[erickench]

Okay I see the problem. If the fault is on the supply side of the disconnect then the circuit breaker would not sense it. It won't trip. Sorry.:grin: