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#129598 - 05/16/05 11:53 PM Dual services... 130-140V to ground?  
u2slow  Offline
Joined: Sep 2003
Posts: 200
Salt Spring, BC, Canada
I'm working in a building with a 240V 3Ø service for 'power', and a 120/240V 1Ø service for 'lighting'.

There are welding plugs (240V 1Ø) run off the 'lighting' service. The strange part is I get ~130V from one leg to ground, and ~140V from the other leg to ground.

I speculating the POCO is using a delta 4-wire arrangement to supply the two services.

Is my voltage-to-ground reading indicative that the welding plug is not properly bonded? (i.e. measuring delta-system capacitive 'ground' instead?)

I ran out of time on this visit... I'm going to delve into it further next time.

Tools for Electricians:

#129599 - 05/17/05 05:05 AM Re: Dual services... 130-140V to ground?  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Hello, u2slow;

I have dealt with many Dual Metered Services, such as the ones you are describing.
One Gear Section (w/ KWH Meter) for 120/240 VAC 1Ø 3W "LIGHTING",
and another Gear Section + KWH Meter for 240 VAC 3Ø 3W "POWER".

Most likely, the PoCo's Transformer bank is a 4 Wire Delta - could be an Open Delta, or a Closed Delta configuration used - I have seen them both used, depending on the location.

I have seen a few locations with 3Ø 3W "Power" as a 3 Wire Closed Delta arrangement, and the 1Ø 3W "Lighting" from a single phase Transformer, for Customers with dual Services.

These were in areas which were primarily Commercial / Light Industrial, whereas the 4 Wire Deltas were in Commercial / Residential type areas, but seen plenty of 4 Wire Deltas in Commercial / Light Industrial areas too.

The 4 Wire Deltas were Pole Mounted Configurations, consisting of:

[*] On Open Deltas: very large 1Ø Transformer, with all secondary taps used, and a smaller 1Ø Transformer used to create the 3Ø "C" leg,

[*] On Closed Deltas: Center Xformer used for 1Ø 3W is maybe 200% + the size of one of the two Outer Xformers.

The 130-140 VAC reading may be either from the type of Meter used, or simply what is being delivered from the PoCo at that time - like at a low usage time, or maybe the P.F. is Leading (Capacitive Reactance).

The 140 VAC is reaching the "Excessive" limits, but should not be too concerning unless there is Equipment which does not like that high of Voltage - and this is also measured at the Equipment during operation.

Let us know what you find after the next visit.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#129600 - 09/03/05 10:33 PM Re: Dual services... 130-140V to ground?  
gerry  Offline
Junior Member
Joined: May 2005
Posts: 4
I am not an electricity engineer or any expert.

I live in a place where the mains voltage is 220 ac. Sometimes I have a device requiring 110 ac. I learned this trick from the local electrician, namely, use one of the two wires from the mains and substitute a connection to a metal rod driven into the ground for the other wire from the mains outlet, thereby acquiring 110 ac for the 110 ac device.

How does this setup work?

Suppose you get one wire from the mains and touch it with the metal rod driven to ground, will you set off a short?


#129601 - 09/05/05 12:03 PM Re: Dual services... 130-140V to ground?  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Bill, Sorry to sound like I am answering a DIY question, but this post has been up for a couple of days, and it is just too darn dangerous to let slide.


The 'solution' that you just described will not work in many cases, will be unreliable in those cases where it does work, and is extremely dangerous. Depending upon the circumstances you will do nothing more than kill a few worms and damage your electrical equipment, to triggering ground fault protection equipment, to getting someone killed.

It might work in some circumstances, but far more likely it will damage the connected electrical equipment, because the '120V' is not well regulated. You are connecting electrical power to a non-bonded grounding electrode, which means that any current flow will have to somehow get through the earth back to the source transformer. This current through the Earth could do it's own damage, and also present a considerable shock hazard.

Just don't do this.


#129602 - 09/06/05 09:08 PM Re: Dual services... 130-140V to ground?  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
To add to Jon's post, many (if not all) 220v power areas have one end of the secondary grounded, meaning the ground rod would be using the earth as a voltage-divider resistor.

If this is true, your resulting voltage could be anywhere between 220 and 0. If I am wrong, you'd be getting an unreliable voltage anyway, because earth resistance is not a constant.

In either case, the voltage applied to the load will vary with the current usage (or, more accurately, the load impedance as it attempts to vary the current). You can also create dangerous voltage gradients in the earth.

Bottom line: it's a bad, bad idea.

Larry Fine
Fine Electric Co.

#129603 - 09/06/05 09:11 PM Re: Dual services... 130-140V to ground?  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
U2, are your voltage measurements to earth or the system neutral? The should be the same, of course, meaning zero difference between ground and neutral.

It sounds like you're getting a high voltage from the utility, which will accelerate the power meter fro most load types. ($)

Larry Fine
Fine Electric Co.

#129604 - 09/06/05 11:48 PM Re: Dual services... 130-140V to ground?  
u2slow  Offline
Joined: Sep 2003
Posts: 200
Salt Spring, BC, Canada
I still haven't been back to investigate. [Linked Image]

I checked from hot to ground (metallic oulet box) at the 6-50R welding receptacle and got ~140V. Same on each leg. I confirmed it was fed from a single-phase panel. I got ~120V to ground (panel chassis) on each leg at the panel itself. [Linked Image]

The walls are finished in between the outlet and panel. Judging by the connector in the box, it is piped with EMT - and there's no bond wire in the pipe.

Supposing the EMT is not providing a bond path anymore - perhaps through mechanical damage or dissolved in the slab - isn't that the same as turning your 4-wire delta back into a 3-wire (ungrounded) delta? And isn't ~140V (240/(root 3) what you should read from phase-to-ground on an ungrounded delta system?

#129605 - 09/07/05 09:19 AM Re: Dual services... 130-140V to ground?  
Jps1006  Offline
Joined: Jan 2004
Posts: 615
Northern IL
Is it a true RMS testor? My bro went head to head with PoCo at mt uncle's shop over a funky voltage read. When it turned out it was his cheap testor, he tucked his tail between his legs and scurried over to the supply house to drop $500+ on a nice new Fluke.

#129606 - 09/07/05 09:24 AM Re: Dual services... 130-140V to ground?  
JBD  Offline
Joined: Jul 2001
Posts: 599
The presence or absence of a feeder/branch grounded conductor does not change the system. The creation of an ungrounded or grounded system can only take place at the source of the system which is either a transformer secondary or a generator output.

What are your measured line-line and line-ground voltages at the "power" service and the "lighting" service. Ideally you would take these mesurements as close to the same time as possible.

Can you put a recording volt meter at the load to see how the voltage changes over the course of the day?

#129607 - 09/07/05 06:07 PM Re: Dual services... 130-140V to ground?  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Remember that 'ground' potential is _not_ the same everywhere. The two supply legs might be at 120V relative to the supply ground reference, but at a different voltage relative to a different ground reference.

This would require both 1) bad bonding between the supply and the point that you are measuring and 2) something introducing current into the ground so that there is different ground potential at your measurement point. Perhaps there is a high impedance ground fault on the third leg, dumping current into some grounded electrode, presenting a shock hazard and killing a few worms, but not anything that anyone has yet noticed [Linked Image]

Perhaps the panel at the source is properly bonded to neutral, but the grounding electrodes have dissolved, and the bond to the receptacle box is also toast. In this case your source is only kinda-sorta grounded, through the impedance of whatever ersatz 'grounding electrodes' happen to be bonded to the panel. In this case, 'ground' at the panel will be right at the center tap potential, but elsewhere in the system, the potential to ground might not be as well defined.


If there is bad ground bonding between the source and the point of measurement, then 'stray voltage' could

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