Ground fault relay setting
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Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
I would contact the manufacturer of the product in use and have their technicians set the unit. Maybe you can get some info from the tech. I witnessed the setting and testing of a large service a couple of weeks ago. The tech was very well educated and I had a good time asking questions... after he developed some trust in me.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
There is no easy answer.
The code doesn't allow the GFP to be higher than 1200A.
Depending on the application, the main gets set at 1200A with a significant delay (limited by code) and the downstream is set tight to the main.
Some like to set the GFP as 10% of the phase trip.
It depends on the load types and the critical nature.
Since GFP is on for personnel protection, a 400A trip or 1200A trip for the main would still probably result in equipment failure and downtime, but not catastrophic failure through the enclosure.
The code doesn't allow the GFP to be higher than 1200A.
Depending on the application, the main gets set at 1200A with a significant delay (limited by code) and the downstream is set tight to the main.
Some like to set the GFP as 10% of the phase trip.
It depends on the load types and the critical nature.
Since GFP is on for personnel protection, a 400A trip or 1200A trip for the main would still probably result in equipment failure and downtime, but not catastrophic failure through the enclosure.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
The manufacturer ships these devices set at the lowest settings (IMO to minimize liability) and will offer no settings other than to contact the design engineer.
The design engineer or engineer that performed the coordination study should provide this information. I doubt this information will be forthcoming from the engineers, just my expierence.
Basic rule of thumb is avoid setting the device because in a court of law with excessive damage you will be held at least partially libel.
BUT....STRICTLY MY OPINION...The minimum setting should at or above 400 amps (for a main sub feeders are differnet). The reason for this is the majority of circuit breakers in a typical building are 20 and 30 amp by setting at 400 amp, .1 of a second you will minimize nusiance tripping from a 20/30 amp branch circuit fault. From there I was always told 20%-30% of the OCP device. 2000 amp OCP 400-600 amps, 3000 amp OCP 600-900 amps, keeping the time setting low....
Coordinating or trying to coordinate the GFP with chillers and other large loads is difficult due to the ampacity of the OCP. This can be accounted for best max current setting and higher time delay settings.
BUT....IMO opinion if you need to adjust the setting:
1. Explain to the end user the benefits and draw backs to higher settings.
2. Get them to sign off on the changes.
3. Best if an engineer determines the setting's, but he'll probably ask you for recommendations.
4. Avoid liability.
The design engineer or engineer that performed the coordination study should provide this information. I doubt this information will be forthcoming from the engineers, just my expierence.
Basic rule of thumb is avoid setting the device because in a court of law with excessive damage you will be held at least partially libel.
BUT....STRICTLY MY OPINION...The minimum setting should at or above 400 amps (for a main sub feeders are differnet). The reason for this is the majority of circuit breakers in a typical building are 20 and 30 amp by setting at 400 amp, .1 of a second you will minimize nusiance tripping from a 20/30 amp branch circuit fault. From there I was always told 20%-30% of the OCP device. 2000 amp OCP 400-600 amps, 3000 amp OCP 600-900 amps, keeping the time setting low....
Coordinating or trying to coordinate the GFP with chillers and other large loads is difficult due to the ampacity of the OCP. This can be accounted for best max current setting and higher time delay settings.
BUT....IMO opinion if you need to adjust the setting:
1. Explain to the end user the benefits and draw backs to higher settings.
2. Get them to sign off on the changes.
3. Best if an engineer determines the setting's, but he'll probably ask you for recommendations.
4. Avoid liability.
rcwilson
Senior Member
- Location
- Redmond, WA
ron said:
Ground Fault Protection is mainly to prevent equipment meltdowns from arcing faults and other high impedance faults. Personnel protection is not the primary purpose. Someone contacting a hot 480V lead will not be safer from electrocution on a GFP circuit. It is not like the GFCI protected circuits at 120 V that do provide personnel protection.
But as you point out, a GFP should trip the circuit before the arc melts its way out of the enclsoure, so personnel in the area will be provided some protection. A system with GFP is safer than one without.
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