Ungrounded service
- Thread starter SANG25
- Start date
- Status
- Location
- Massachusetts
You ground and bond the metallic parts just like any other service except that you do not install a main bonding jumper from a line conductor to ground.
charlie
Senior Member
- Location
- Indianapolis
Don't forget, "250.21(B) Ground Detectors. Ungrounded alternating current systems as permitted in 250.21(A)(1) through (A)(4) operating at not less than 120 volts and not exceeding 1000 volts shall have ground detectors installed on the system." :smile:
beanland
Senior Member
- Location
- Vancouver, WA
Ungrounded Delta
Ungrounded Delta
Ungrounded delta systems are very poor choice for electric services. If at all possible, center-side ground one side of the delta. (240/120V and 480/240V are common 3-phase 4-wire delta services used by utilities.) I have seen entire services destroyed because the ungrounded delta is not referenced to ground. A surge on the source side of the service transformer will couple by capacitance through the transformer, raising the voltage on the service to 1000's of volts above ground. Boom. Smoke. Flash.
A ground monitor is good but it requires qualified people to know what it means and troublehooting to find the ground is a pain. We had to shut a whole plant down because one small motor developed a ground and in order to find it we had to turn off one thing at a time until the ground went away. Found another similar when rats nested in a disconnect and their crap created a ground on on phase. Also had an entire lumber mill burn down because two grounds appeared on opposite sides of the plant causing a short that went through the equipment. Not enough current to blow a fuse but enough to cause a fire.
In my opinion, better to blow a fuse.
Ungrounded Delta
Ungrounded delta systems are very poor choice for electric services. If at all possible, center-side ground one side of the delta. (240/120V and 480/240V are common 3-phase 4-wire delta services used by utilities.) I have seen entire services destroyed because the ungrounded delta is not referenced to ground. A surge on the source side of the service transformer will couple by capacitance through the transformer, raising the voltage on the service to 1000's of volts above ground. Boom. Smoke. Flash.
A ground monitor is good but it requires qualified people to know what it means and troublehooting to find the ground is a pain. We had to shut a whole plant down because one small motor developed a ground and in order to find it we had to turn off one thing at a time until the ground went away. Found another similar when rats nested in a disconnect and their crap created a ground on on phase. Also had an entire lumber mill burn down because two grounds appeared on opposite sides of the plant causing a short that went through the equipment. Not enough current to blow a fuse but enough to cause a fire.
In my opinion, better to blow a fuse.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The NEC only allows ungrounded systems in loactions where there is qualified maintenance.
If the fault did not draw enough current to operate the OCPD, what makes you think it would have on a solidly grounded wye system? Single phase to ground faults at 277V are notorious for causing fires, that is why ground fault protection is usually required by the NEC.
The most common problem I have seen with "ungrounded" systems is the lack of proper equipment bonding. With proper bonding the first connection to ground simply changes the ungrounded system into a phase grounded one.
shortcircuit2
Senior Member
- Location
- South of Bawstin
240 volt delta
240 volt delta
What kind of voltage readings would you have between any leg of a 240volt ungrounded delta to ground? Lets say with a pair of wiggy's or fluke digital meter...
I'm not that familar with this type of service and an associate called because he has 240 volt+/- to ground on phase A+B ...but 0 volts to ground on phase B.
This is a Bank with the 3phase service for the AC units only and another service for lighting, etc...
shortcircuit
240 volt delta
What kind of voltage readings would you have between any leg of a 240volt ungrounded delta to ground? Lets say with a pair of wiggy's or fluke digital meter...
I'm not that familar with this type of service and an associate called because he has 240 volt+/- to ground on phase A+B ...but 0 volts to ground on phase B.
This is a Bank with the 3phase service for the AC units only and another service for lighting, etc...
shortcircuit
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
Ungrounded service
My wiggys tester shows no voltage but my flute digital showes voltage 130 +or-. I try a 120v lightb blub from phase to ground it dosen't light. Short out phase to ground and it doesn't trip the breaker.i can feel voltage on my finger from phase to ground. Any ideas
My wiggys tester shows no voltage but my flute digital showes voltage 130 +or-. I try a 120v lightb blub from phase to ground it dosen't light. Short out phase to ground and it doesn't trip the breaker.i can feel voltage on my finger from phase to ground. Any ideas
zog
Senior Member
- Location
- Charlotte, NC
That and ignoring the pressence of the first ground.
- Location
- Massachusetts
It sounds like you have a fully functioning ungrounded service, all the things you describe indicate so.
I do not recommend using your finger to check for voltage.
zog
Senior Member
- Location
- Charlotte, NC
Yeah, I have some ideas, either learn some new T/S skills or get someone qualified to look at this for you.
3 Phase 3 wire Services
3 Phase 3 wire Services
I have worked for Department of Water and Power as a lineman cable splicer for 30 years. I have seen this type of 3 wire 3 phase service before. It was very common in the past. The transformer hook ups were delta - delta. Because there is no relationship to ground in a true Delta you had to ground one of the phases. This was called a "corner ground" or "Scott Ground". But I have also seen a Delta-Delta wired transformers hooked up to a machine shop and one of their machines had a ground on one of the phases. To make matters worse the transformer grounds were weak. When the machine went to ground the whole system acted like a Scott Ground, but it wasn't! To find this I had to open one machine disconnect an a time until the corner ground cleared. I then drove new good grounds and attached to the existing transformers and told the shop owner to repair his equipment. If he had closed the disconnect to the motor after I repaired the transformer grounds there could have been a fire or just a tripped breaker. ---- I checked back later and the motor was replaced.
3 Phase 3 wire Services
I have worked for Department of Water and Power as a lineman cable splicer for 30 years. I have seen this type of 3 wire 3 phase service before. It was very common in the past. The transformer hook ups were delta - delta. Because there is no relationship to ground in a true Delta you had to ground one of the phases. This was called a "corner ground" or "Scott Ground". But I have also seen a Delta-Delta wired transformers hooked up to a machine shop and one of their machines had a ground on one of the phases. To make matters worse the transformer grounds were weak. When the machine went to ground the whole system acted like a Scott Ground, but it wasn't! To find this I had to open one machine disconnect an a time until the corner ground cleared. I then drove new good grounds and attached to the existing transformers and told the shop owner to repair his equipment. If he had closed the disconnect to the motor after I repaired the transformer grounds there could have been a fire or just a tripped breaker. ---- I checked back later and the motor was replaced.
glene77is
Senior Member
- Location
- Memphis, TN
Sang2,
Here is an idea:
Wet your finger before using it like that again.
That will remind you what a Wiggy is for.
You are reading 'phantom voltage' with the Fluke because the imput stages are hi-impedance (maybe 10 Meg Ohms).
The Wiggy will have about 3Kohms impedance, and draw about 40 milliAmps.
I have found that this small load will kill the phantom voltage caused by capacitance between conductors.
Here is an idea:
Wet your finger before using it like that again.
That will remind you what a Wiggy is for.
You are reading 'phantom voltage' with the Fluke because the imput stages are hi-impedance (maybe 10 Meg Ohms).
The Wiggy will have about 3Kohms impedance, and draw about 40 milliAmps.
I have found that this small load will kill the phantom voltage caused by capacitance between conductors.
- Status