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#127919 03/13/02 10:18 AM
Joined: Mar 2002
Posts: 1
Junior Member
I have started a new job in maintenance in a metal forming plant. They tell me not to trust my dvm. when testing ohms because they have a floating ground. What is this? I have looked through the books and can not find anything about floating ground.

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#127920 03/14/02 10:38 PM
Joined: Dec 2000
Posts: 218
pannell, I too work maintenance in a metal forming plant. What are you ohming that you are told not to trust a DVM? Could it be that the control circuit xformers are not grounded on the secondary? This causes some strange Voltage readings on control circuits. I'll check back on this post this weekend.

[This message has been edited by spkjpr (edited 03-14-2002).]

#127921 03/15/02 09:34 AM
Joined: Mar 2001
Posts: 2,056
When using an ohm function, the main point is to remove at least one end of the component you are testing from the circuit. I don't see what a floating circuit could mean to an ohmmeter, unless you are looking for continuity to ground.

#127922 03/15/02 09:06 PM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
I have no idea where the term "Floating Ground" would have bearing on a test like you are describing.

As pointed out by others, if there's any components connected while you perform any continuity / resistance tests, they will alter your readings.

When you do AC Voltage tests with a High Input Impedance type DVM, this might have inaccurate readings. The most common is reading Voltage to Ground on an Ungrounded system.
In contrast, continuity to ground [Ohms] may result in an inaccurate reading on the Ungrounded system, since their will be a certain level of Capacitance to grounded equipment [the Coupled effect which causes the High Z DVM to give various AC Volt readings from Line to Ground], or even to ungrounded metallic equipment.

Not too sure what the term "Floating Ground" would be best described as - other than someone installing equipment where there is no continuity of the Equipment Grounding Conductor / System [seen this a few dozen times!].

Scott SET

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#127923 03/18/02 05:25 PM
Joined: May 2001
Posts: 377
Could be a he means that there is a problem with the grounding system,potencial to ground may be there one minute and gone the next so don't trust the circuit is de-energized while checking with your meter to ground.Could also be that there are some LIM panels or other weird isolated ground systems around the plant but if they know they have floating conduit or ground wire connections i would think they would do something about it.

#127924 03/19/02 03:56 PM
Joined: Nov 2001
Posts: 280
Would the following qualify as a floating Ground ??????????
I finished a job at an old apartment complex today, small job couple of GFCIs for kitchen and a 50 range run.
it has 5 apartments and a house panel all fuse boxes. I was working in apartment two, it was empty and so decided to pull the plug for apartment since it was daylight and I was working in the kitchen and had plenty of light.
Pulled the main fuse block and checked the panel and all the associated circuits and all was fine. went to the apartment and commenced to pull out two of the 3-wire receptacles, (this should have been my first tip-off), got them out and got ready to remove the wires when I got Zapped, not a real strong Zap associated with 120volts but good enough to let me know I had been zapped. Check the voltage again, hot to white it was 0 volts . Cchecck the voltage to ground the meter said 66.5 volts. Upon further inspecction there was a black wire exiting the box, this was tied to the grounding screw of the receptacle and into the wall, looked under the sink and sure enough it was tied to the water line. (ala 250-130 older codes)
Looking further into this I found in the craw space every single old two-wire cable had a black piggy-back wire attached to it. The whole problem is the interior water lines are only part of the building, the water system is a well which is well insulated from the copper piping by the plastic incoming water line. plus these lines were never bonded to the service.
Has anyone else ever seen anything like this ?

#127925 04/04/02 04:40 AM
Joined: Apr 2002
Posts: 7
Junior Member
this is called a floating neutral.the transformer is not grounded and with the power turned of the branch circuit,the neutral will still have full voltage on it.very dangerous.

#127926 04/04/02 08:46 PM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
Actually, the Grounded Conductor [Center Tapped "Neutral" in 1 phase 3 wire, or Common in 3 phase 4 wire Wyes] always has Voltage on it. On a 120/240 VAC 1 Phase 3 wire system, the Grounded Neutral Conductor is 120 VAC [there is 120 VAC between it and one of the two Ungrounded Conductors].

If the Center Tap or Common was not Grounded at the Transformer + Service, this will not make the system Dangerous - just Ungrounded.

Take the 3 wire Delta for example:

If the system is "Corner Grounded", one "Phase" is Grounded at the Transformer + Service and the system has a Grounded Conductor. This system will draw high currents on either Ground Faults or L-L / L-L-L Faults. The Voltage to Ground is stabilized on this system.

If the system is Ungrounded, there is no Grounded Conductor or bond between the system and the Metallic Raceways, or the Earth it's self.
A Ground fault will draw very low current levels. L-L / L-L-L faults will draw large current levels.
The Voltage to ground is not stable, as it would be on a solidly Grounded system.
This means that the Voltage from a given Conductor and at different points in the system can vary when applied across Impedances above 1K ohm.
This will kill a person, but not even come close to tripping an OCPD.

Just wanted to mention this.

Scott SET

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

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