Monitoring Total Residual Current

[dkerwin]

I have been doing alot of research on the cause & effect of stray voltages, and here is a summary: If stray voltages exist within a residence or business, then a portion of the current that should be returning through the neutral path is actually flowing through a different path - most likely via bonding /grounding connections back to the source - and due to some sort of insulation breakdown.

There are literally hundreds of regulations published by the NEC, NESC and OSHA to ensure a relatively consistent and substantial path from each electrical load back to the neutral connection on the utility?s meter. All involved with electrical work in residential and commercial installations are reminded over and over that the ground was never intended to be (nor should be wired as) a return path for neutral current.

So, why do residual (leakage) currents still exist? A simple answer is that the codes were not followed, and that existing circuit connections were somehow overlooked or 'grandfathered? as newer codes were established.

That being said, wouldn't it be prudent to measure the total residual current (TRC) of a residence or business 'after' the utility meter to get an idea of the baseline value? Then, monitor that quantity over a day, week and month to see if there are changes (loss of insulation) due to switching loads, weather, etc.?

A few other questions:
1) Does anyone know whether the utility is responsible for the level of TRC existing within a facility?
2) Has TRC ever been used as part of a pass/fail test during any electrical or building inspections?
3) Has anyone ever used TRC as a tool for identifying faulty electrical or mechanical installations? (i.e. drywall screw or stud nail semi-piercing cable insulation).
4) Is there a maximum level of TRC which could be considered unsafe for residents or employees of a facility?

Just curious......

 

[Guest]

Re: Monitoring Total Residual Current

Two Words: Karl Riley

Tracing EMFs in Building Wiring and Grounding

 

[al hildenbrand]

Re: Monitoring Total Residual Current

You raise some interesting questions.

path from each electrical load back to the neutral connection on the utility?s meter

Did you mean from the load back to the grounding electrode system (GES) to neutral bonding point in the service?

On the line side of the GES/neutral bond, the load return current has multiple return paths available to it by Code requirement. Any neutral current that returns to the source will split and take all available paths in an amount inversely proportional to the path's impedance.

On the primary side of the distribution transformer, any unbalance current on the "neutral" is also presented multiple paths at each re-grounding point along the transmission line, as well as the interconnection with the transformer secondary neutral.

And, also, the neighboring occupancy that shares a non-electrical system that is conductive (think municipal water supply distributed in metal pipe) with the occupancy you are observing, will contribute part of its neutral current to the path through the water pipe and out through the service drop.

There is a rich soup of currents on the line side of the GES/neutral bond of an occupancy that is unrelated to the load in the occupancy.

 

[brian john]

Re: Monitoring Total Residual Current

KW:
The firm I work for and the previous firm I worked for did grounding investigations. Most are conducted only after a PQ problem arises. GFP trips or there is screen shake, equipment hum etc.

There is a lot of strange (at least to date to me) currents I have found in facilities. I have two different facilities with 277/480 VAC 4 wire wye services, the neutral/grounded conductor does not leave the service all loads are 480 3 wire. Yet we have ground current, have looked at all possibilities, I know to check, obviously I have overlooked something and I will find it.


BUT---------I am very interested in conversing with anyone that is involved in ground current/stray voltage investigations.

Either in this forum, by email, phone or face to face.

 

[dereckbc]

Re: Monitoring Total Residual Current

Dkerwin, you have identified some of the means of current on grounding conductors, but may be overlooking some of the less obvious sources. I have performed many PQ audits over the years in the Telecom sector, and we trigger an audit when there is more than one-amp flowing on the MBJ.

What I have found over the years are some of the causes you mentioned such as improper bonding of the neutral and insulation problems. Some sources are rectifiable and some you have to live with. Here is a list of some of the most common items I have found.

1. Cable capacitance. In large facilities the capacitance between L-EGC, L-Conduit, N-EGC, and N-conduit will inject small amounts of 60 Hz load current onto EGC and raceways. If you look at it with an oscilloscope, you will see 60 Hz along with a greater amount of harmonic currents. The higher the frequencies the lower the capacitive impedance.

2. TVSS devices connected in all modes. The modules connected from L-G and N-G will leak current in to the ground circuits. The larger the facility is with large numbers of TVSS device compounds and accumulates the current.

3. Electronic and electrical equipment with RF filters installed from L-G and N-G rather than L-N will inject current into the ground circuits.

4. Various line reactors and harmonic filters installed in a facility.

5. -48 VDC power plants installed in some facilities that power communication equipment will supply equipment with the battery return electrically connected to the chassis will induce DC current to flow in all ground circuits.

6. HID and lighting ballast.

7. And finally lets not forget about our local POCO and the way we have to terminate the grounded circuit conductor. If you use a facility that employs multi-point grounding methods (such as a grid or ring), or lightning protection systems current is going to flow in your ground circuits. Ask any rancher.

So IMO current flowing in ground circuits is a way of life unless you take extreme measures to prevent them, which is not a requirement. So all you can do is eliminate the ones you can deal with and ignore the rest.

 

[tom baker]

Re: Monitoring Total Residual Current

dkerwin
Awwt has given you a very good recommendation. Karl is an inactive moderator at the moment, and Derek has very capably taken his place. I do not have Karls book but I am confident it is very well done.

 

[dkerwin]

Re: Monitoring Total Residual Current

Thank you for all of the feedback. However, I still haven't obtained the answer I am looking for.

I am not so concerned about what the actual codes require, or where ground faults can possibly come from, or even how to find the many causes. What I am concerned with is the loss of insulation on electrical equipment, the associated risks of electrocution that result, and whether monitoring TRC (L1 + L2 - N currents) within a residence or business could be used as an effective tool in detecting them?

Example: My sump motor has developed a semi-high resistance short-to-case. Now, during operation, some of the 'load' current is flowing via the short (ground fault) back to the source through the many ground connections. If the impedance of the ground path is low enough, there could eventually be enough voltage developed on the motor case to cause more electrical damage - or even electrical shock.

A GFCI or AFCI would pick it up. However, the code doesn't require these devices on every load. Plus, in this case, you may not want to necessarily trip, but rather monitor & investigate.

If there were a device monitoring TRC, and you could detect a potential (developing) ground fault, wouldn't it prove to be an effective tool in preventing potential hazzards?

 

[dereckbc]

Re: Monitoring Total Residual Current

Originally posted by dkerwin:
If there were a device monitoring TRC, and you could detect a potential (developing) ground fault, wouldn't it prove to be an effective tool in preventing potential hazzards?

Yes. In this case I would think a simple current probe on the MBJ would be more effective. In fact I have used such devices connected to a power monitor system. I connect them to the MBJ and GEC.

[ November 17, 2003, 04:53 PM: Message edited by: dereckbc ]

 

[karl riley]

Re: Monitoring Total Residual Current

Dwerkin,
Your example of insulation failure is a relatively rare cause of neutral-to-ground connections on the load side of the service disconnect. Much more common are N/G bonds in subpanels, nicked neutrals in fluorescent fixtures, intentional N/G jumpers on receptacles, and a number of others.

These residual, or "net" currents will be on the individual branch circuits, so your detection device would have to be on each circuit separately. The easiest detection device is a gaussmeter, since you turn it on and you see the consequent field. A clamp-on ammeter around the circuit will show any net current. A GFCI on each circuit might be the cheapest prevention, but some circuits are not suitable. In commercial facilities doughnuts (like a permanent clamp-on)can be attached around each circuit but this can be expensive for residential.

Hey guys, I just got back from two trips, so I will be chiming in from time to time.

Karl

 

[karl riley]

Re: Monitoring Total Residual Current

Oops, it's Dkerwin. Sorry!

And Dereck knows about the monitoring setups.

Karl

 

[brian john]

Re: Monitoring Total Residual Current

Dereck:

I have visited data/telco sites where the neutral was 100% properly installed, all possible sources of ground current was investigated yet, there was significant ground current, not on the neutral ground bond connection, but flowing in the EGC's. At one facility we measured 36 amps (on the EGC's) at places but only 2.35 amps on the Neutral ground bond. This particular site we never did resolve the issue as the firm went belly up (DOT COM bust), we never got paid and they weren't worried about the problem, as they were busy sending out resumes.

 

[dereckbc]

Re: Monitoring Total Residual Current

Brian John, was it AC or DC?

 

[al hildenbrand]

Re: Monitoring Total Residual Current

The common EGC forms a web of interconnections from circuit to circuit from the load back to the neutral-grounding electrode system (GES) bonding point, with an additional large random component introduced by multiple incidental connections of uncertain quality to conductive mechanical and structural systems (non-EGC). This non-EGC conductive "field" (if you will) can extend way beyond the apparent physical confines of electrical service to load wiring assembly, and is likely, in my opinion, to interconnect with the neighbors.

Your observation of 36 amps on EGCs (I note you used the plural) yet not appearing at the GES/neutral bond, to me, begs the question of what the condition of the neighbor's electrical systems are like.

I will be mystified (save if this is DC) if you tell me the Dot Com was in the middle of a corn field, miles from any other facility.

 

[brian john]

Re: Monitoring Total Residual Current

The dot com was in a 6 story warehouse shared with other dotcoms (colo hotel). The other dotcoms were not in service at this time though there was some load. Each tenant had a 13.2 delta to 480/277 4 wire wye. This tenant utilized three wire distribution tough they did for some reason pull a neutral to distribution gear through out the facility. Lighting was from 480 delta to 480/277 wye XMFR’s. While I did not check the other tenant services the contractor that hired us had and said no current was noted, we could not gain acces to these tenants while we were on site.

The 36 amps was a 60 Hz sinewave (scoped), did not have PQ equipment to check for harmonics. Did check for DC current there was none.

The current was proportional to the load in the facility. As we shut down loads the GEC current reduced. The current at no load was .36 amps (basically Zero).

Loads were two 750 KVA UPS's, 2200 amp DC distribution at 48 VDC (nominal), lighting loads and HVAC loads. The UPS and DC power did not have any significant loads additional loads were on at this time.

 

[brian john]

Re: Monitoring Total Residual Current

The service fed two separate distribution gears one for critical power (UPS's) one for mechanical loads, rectifier loads and lighting. The feeders left in parallel conduits ran over top one another, the gears we set one in front of the other 150 feet away.

We could vary the EGC current depending on how we connected the EGC's in the main gear. The Main ground bar in the main gear was a U shaped copper bar with the feeder distribution in the middle of the U.

I was working for an electrical engineering firm and a dotcoms/Telco power engineer. I suggested the current might be induced due to site design and switchgear design, I was met with a resounding YOUR NUTS we paid you for this. OH yeah I never got paid, the DOT went belly up and they knew this was coming when they hired me. The dotcom was hoping to resolve this issue so they could sell the site to get needed capital.

While I did want to get paid, I also wanted to get back to the site to try to resolve this issue. I hate leaving unfinished business, especially where my skills/knowledge are questioned.

 

[dkerwin]

Re: Monitoring Total Residual Current

brian john: You mention a DC system at the Telco (2200A/48V). Have you had any experience with ground detection in those systems? When there is a ground, how do they detect it and find it?

 

[dereckbc]

Re: Monitoring Total Residual Current

Originally posted by dkerwin:
You mention a DC system at the Telco (2200A/48V). Have you had any experience with ground detection in those systems? When there is a ground, how do they detect it and find it?

dkerwin, I have experience with 48 VDC power plants. Sorry Brian John don't mean to hy-jack you question.

Most 48 VDC plants have the positive battery return bus referenced to ground. I have seen a few applications where neither positive or negative busses are grounded. Ground detection is the same as for AC ground detection. The simplest method is two light bulbs connected in series across the positive and negative terminals of the battery busses, and grounded between the two light bulbs. If both bulbs are equally brilliant everything is OK. If one light extinguished while the other gets bright you have a ground fault on the extinguished bulb.

But as I said most plants have one bus grounded. The positive on 48 VDC systems, and negative on 24 and 12 volt systems. Unless the operating company is very stringent about single point isolated ground?planes, and only uses equipment made for that purpose, there will always be DC current flowing on the frame ground conductors. The reason is because a lot of the equipment out there has the positive terminal electrically connected to the equipment chassis. I have seen as much as 330-amps DC flowing on equipment frame grounds on a 12,000-amp 48 VDC battery plant.

 

[brian john]

Re: Monitoring Total Residual Current

Dereck:

I'm sure you've seen contractors wire systems wrong or be totally baffled when they run into 24 VDC neg GND. and 48 VDC pos GND at the same facility. Had more that one contractor call us to resolve faults from wiring the loads improperly.

 

[dereckbc]

Re: Monitoring Total Residual Current

Originally posted by brian john:
I'm sure you've seen contractors wire systems wrong or be totally baffled when they run into 24 VDC neg GND. and 48 VDC pos GND at the same facility. Had more that one contractor call us to resolve faults from wiring the loads improperly.

Yep. I have seen inexperienced contractors swear the whole place would blow up with a 48 VDC positive ground bus, and 24 volt negative ground bus installed right next to it. They never worked for me again. Fortunately, most of the time, I get to use contractors who specialize in DC plant installations. It is a specialized field. Most do not understand all the extra installation techniques and grounding principles required.