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Voltage at the ground rod.

M

Mark Ash Fork Electric

Member
Location
Northern Arizona
Occupation
Electrician
I found 583 volts at the ground rod the hard way as a AC unit started up. Voltage creeping down gradually after breaking rotor.
I have isolated the problem to the AC unit.
It is a 240 vac unit without a neutral.
Conductors and connections are in good condition.
The blower motor recently installed.
I can not think of anything but a short within the compressor.
Which direction would you go?
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
All voltage measurements are between 2 points.

Where were both of your meter probes when you measured 583V.

Does the AC unit have an inverter or is it an across the line compressor?

Have you verified proper electrical system grounding? (Connection between neutral and earth? )

Jonathan
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Turn it over to the Heating/Air guys.

240 volt AC units do not use Neutrals. They should have an EG.

A short within the compressor will trip a breaker.

A winding faulted to the case may cause a shock, if an EG is not properly connected at all. It would need to be a PP install.

First check would be the installation of that blower motor.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Little Bill said:
How do you get 583V off a 240V service?
Click to expand...
Ungrounded source and intermittent fault in a motor, turning the entire electrical system into a boost converter.

(Note: this is very unlikely in a residential setting. Ungrounded here means no bond from neutral to ground anywhere, not a lack OF EGCs.)
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
ptonsparky said:
Why is there a rod at all if this is an ungrounded system? Pole ground for utility?
Click to expand...
The only difference between the grounding and bonding requirements between grounded systems and ungrounded systems is that the grounded system has a main bonding jumper and the ungrounded system does not.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
don_resqcapt19 said:
The only difference between the grounding and bonding requirements between grounded systems and ungrounded systems is that the grounded system has a main bonding jumper and the ungrounded system does not.
Click to expand...
Yes, I'm just having a hard time seeing where a rod would be so close to the equipment that you could contact it alone, accidentally.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
ptonsparky said:
Why is there a rod at all if this is an ungrounded system? Pole ground for utility?
Click to expand...
You still need EGC and bond to a GEC for ungrounded systems.

If it is utility supplied system is pretty unlikely to measure such a voltage, if it is separately derived system it can be possible.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
ptonsparky said:
Yes, I know you need a GES, a bonded EGC, but if you have those how are you getting 583 volts at a rod that is supposedly part of the GRS?
Click to expand...
As mentioned we don't really know what items we have voltage between. For no more than what we know apparently a ground rod (or at least it's connected GEC/bonding jumper) is one point of the measurement. About has to be some capacitive voltage involved unless it maybe is the bad primary neutral situation mentioned.
 
M

Mark Ash Fork Electric

Member
Location
Northern Arizona
Occupation
Electrician
Thank you for your responses. I've had a bit of a delay getting back to this thread, many other jobs in between. I have a residential, 200 amp single phase 240 volt panel. It is feeding a manufactured home, heat pump, and a tool shed. This system is on grid, I deal with a lot of off grid in this neighborhood. The original problems started with voltage in the ground and was frying out a cable TV contactor. They put in their own separate ground rod and fix that problem. Meanwhile I separate the number fouqr ground wire from the ground rod and would find voltage there. Between the number four wire and ground rod. When the heat pump started voltage jump to 583 volts and then settle down to 240 something after a minute. Not paying attention, I went to reconnect the ground wire to the rod just as the heat pump started. Didn't turn that breaker off. And I got the full Voltage from thumb to small finger.
 
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