Objectionable Currents on Grounding Conductors 2

[jwilson3]

NEC 250.6 covers objectionable currents on grounding conductors, but gives no guidance as to what level of current is objectionable. Soares "Book on Grounding" states that it's any current that prevents the grounding conductor from maintaining earth potential at each grounded device. This makes obvious sense, but implies ZERO current, which can't exist because of inductive and capacitive relationships between the grounding and the current carrying conductors.
So, between the small "induced" currents and higher currents that would present a clear shock and otherwise safety problem, what would forum members offer as to "how much is too (objectionable) much"?

 

[buzzp]

For protecting people it shoud be 30mA or less. For equipment or other issues (potential rises etc) it is system dependant but there is likely some rules of thumb concerning this that I just dont know yet.

But even so, each installation would have to be evaluated seperately for possible problems.

 

[RonShap]

Are you referring to the grounding electrode, or equipment grounding conductor?
Since neutral current divides on its way back to the service (or separately derived source) at the neutral to ground bond, part on neutral, part on grounding electrode conductor (current divider).

 

[jwilson3]

RonShap,
The equipment grounding conductor is the one I'm referring.

 

[jwilson3]

buzzp,
Actually the 30mA you refer to is the current flowing thru the trunk of a human body that's at the lower threshold of a fatal shock. It can result when a person contacts a voltage of 30v. So, it's the voltage on the grounding wire that's the "killer" and it's the current in the wire that produces the "killer". 30mA of current in a #12 grounding conductor won't produce 30v of voltage drop without a lot of length.
In my opinion any current that produces enough voltage drop, which is then impressed on a grounded object, that causes even a mild shock is objectionable.

 

[RonShap]

jwilson,
The EGC of what? If it is the equipment ground conductor of a separately derived source, then it will carry its portion of neutral current (current divider) from the neutral to ground bond.

 

[jwilson3]

RonShap,
OK, here's the rest of the story. There's no separately derived source in question here. In the main switchgear there's a neutral bar "bonded" to the grounding bar. The service includes a grounded conductor[the neutral], but no grounding conductor. The grounding electrode conductor -- from the ground bus to the electrode(s) -- has 2A on it. That, in my opinion, is part of the neutral current flowing back to the source thru the earth. The "bonding jumper" has 16A on it and that, in my opinion, is coming from the grounding conductors and flowing back to the source neutral. The measured neutral current was 50A.

I think that is too much ground current. It can't be zero, as there will always be leakage, inductive, and capacitive currents. I can't find anything in the NEC that quantifiably supports my position.

 

[weh3]

J,

16 A on your MBJ is a fire hazard. It appears you have a high-impedance ground fault or other phase imbalance in progress that is bypassing your protection. The protection should encompass the neutral to pick up any zero-sequence current.

Regards,
William

 

[buzzp]

Theres definately issues here. The 30mA is the current trip level for Class I GF protection (personnel). If this was in the circuit it would trip.
There is no threshold as identified from the NEC for normal operations as normally (best circumstances) there is no 'ground' current. But we all know this is generally not the case in any industrial or commercial installation.
Your levels are definately high enough to warrant a priority investigation. Start with finding any old measurements. Its not clear from your description (at least clear to me) that the bonding and grounding is done properly. If it is, 16A on the bond is definate cause for concern.

 

[buzzp]

One more thing, how does the poster get a star?

 

[peebee]

Hmmm. . . . I'm surprised at the concern about the 16 amps. Particularly given that this is "switchgear", which sort of implies ampacities of around 2000 amps and potentially long cable runs.

It may not be a perfect installation, and it may be worth checking that:
- there are no low-level ground faults.
- there are no unintentional N-G bonds.
- that conductors are routed through a single raceway system such that net current through any raceway is zero.
- that your electricians know the difference between neutral and ground and don't mistakenly connect equipment neutrals to the system ground conductor.

But, you might get those levels of ground current even with a fully code-compliant installation. Sources of ground current include:
- EMI
- induction between phase & neutral conductors to ground
- noise filters & surge suppressors
- assymetry in the system.

Unfortunately, I cannot give you a firm answer on just how much ground current is acceptable. But 16 amps at a main switchboard certainly would not bother me too much.