brian john
Senior Member
- Location
- Leesburg, VA
WHY MAIN LINE SWITCHES TRIP:
1. Modern Switchboards have incorporated many optional protection schemes in addition to standard OCPs and GFP there is blown fuse protection, normally 3-KAZ fuses in parallel with the main fuses if the a main fuse blows the parallel KAZ blows and a spring actuator pops up to operate a microswitch resulting in operation of the Main Line Switch shunt. In addition the following or a combination of the following may be installed, phase relays, under voltage relays, and reverse phase relays to offer loads on the distribution system protection.
2. So prior to chasing a Ground Fault, one has to determine if the Switch operation was a result of a blown fuse, UV or phase loss. While the blown fuse operation is simple to find, it is obvious when you look at the KAZ and additionally some Switchboards have blown fuse indication lights on the front of the Switchboard. Phase loss and UV operation can be more difficult to diagnose, if the power is restored when you arrive on site.
3. Some GFP relays have indicators that will not allow you to close the Main Switch without resetting the indicator, others allow you to close the Main Switch with the indicator still showing a GFP operation and other GFPs systems have no indication of operation, leaving you to guess the reason the Main Switch opened.
4. In order of operation based upon personnel experience for Main Line Switch opening.
a. Electricians tracing circuits short a 408/277 VAC branch circuit, or electricians or others as part of construction short a 480/277 VAC circuit. Electricians that are still tracing circuits in this method are NOT electricians. *1 see below
b. Faulty equipment shorts to ground; number one here is motors, with cooling tower fan motors being number one, HVAC compressors number two. *2 see below
c. An actual ground fault with water leading the list for the cause of the ground fault.
d. A defective GFP relay or other protection device, defective CT or an open in the CT secondary wiring.
e. A fault in 208/120 VAC distribution in a properly installed system will not operate a 480/277 GFP
5. *1 In a typical office building the majority of branch circuit breakers are 20 amp and 30 amp, if the GFP relay is set at the 100, 200, or 300 amp setting a fault on a 20 or 30 amp branch circuit can result in an operation of the Main Line Switch. A standard molded case circuit breaker will operate in the instantaneous range at 6 to 10 times the rating of the CB 10X20=200 or 30X10=300, plus or minus 35% accuracy. So if the GFP relay is set low a majority of the branch circuits can result in a GFP operation. IMO this is not good safe coordination, I feel 400 amps should be a minimum (BUT I AM NOT AN ENGINEER).
6. *2 with motor faults and large equipment faults it is almost impossible to coordinate the GFP with the OCP for the utilization equipment. Assuming the same 6-10 times instantaneous rating of the OCP a 100 amp OCP would operate on a ground fault at around 1000-1200 amps which may be at or above the GFP relay settings (maximum setting 1200 amps).
HOW TO SAFELY RESET GFP PROTECTED MAIN LINE SWITCHES:
1. Try to determine if the Switch opened from GFP operation or other system protection device.
2. Ask if there is any on going construction.
3. Ask if the Switch opened at dusk, outside lighting can be an issue especially after periods of rain.
4. Check the GFP settings time and current, write them down.
5. Have someone (because it requires a lot of walking) shut off all downstream devices at the level below the Main.
6. Once you are sure the power is off, open and test the bus with a tester you are sure works (HAVE YOU TESTED YOUR TESTER). It is possible for Switches to hang up BECAREFUL.
7. Inspect the bus for any water and signs of a fault.
8. Megger the bus phase to phase and phase to ground, if the reading is low check that there are no relays connected to the bus, bus taps or other loads you are unaware of. Normally we test the bus at 100 VDC first to avoid damaging protective relays such as Phase or UV relays the retest at 1000 VDC. We?d like a reading above 50 megohms, 5 can be acceptable, .5 as recommended by some IS NOT ACCEPTABLE, IMO.
9. Open the next downstream OCP and then megger the feeders. Continue in this manner.
10. With HVAC equipment, motor loads, large duct heaters and other utilization equipment you will need to get to the load conductors for the equipment, do a visual on the equipment looking for signs of a fault.
11. Check for blown fuses and opened CBs as you work your way through the building.
12. Hopefully you will located the source of the fault, if you don?t the problem may be with the relay and require testing, and this is more than just pushing a button. A high current test set is required.
13. If you have done all of the above, with exception of item 12, you may have to close the Main Switch without resolving the issue, THIS IS CONTRAVERSAL and may violate local and federal rulings, BUT IMO is sometimes the only solution when all avenues have been exhausted.
a. Set the GFP relay to 100 amps (or lowest setting) and the time delay to the lowest time setting Instantaneous, .1 or minimum are typical minimum settings.
b. READ THE SWITCH OPERATION Instructions, the number of Service calls we receive for damaged Switches due to improper operation exceeds a level that would be obtained if Electricians READ THE INSTRUCTION.
c. With all downstream OCPs open, all covers installed on the Switchboard, all cover screws installed tight, you dressed in proper safety gear and no one else in the room; close the Main Switch in such a manner that places you in as safe a position as possible.
d. If the Switch holds and it should if you have done all the above steps as noted. Move on to the next level OCP.
e. Close these devices one at a time allowing any equipment that may need to start time to start.
f. If the GFP operates at any point and you gave equipment sufficient time to start you may have located the fault.
g. Reclose the Main Switch per step 13.c. and start closing all the OCPs leaving the one with the fault off.
h. Bring the building on line and return to the OCP with the suspected fault and isolate the source of the fault.
i. One issue to be aware of is; large dry type transformers have inrush currents that can trip GFPs relays due to the inrush current are not Zero Sequence.
j. Once you have isolated the fault, remember to reset the GFP settings to the ?AS Found Settings?, reset the GFP indicator (if present).
14. If after this you have not located any problems, the GFP relay should be left set at the lower settings, the GFP tested at all presets for current, and if all checks out it may be necessary to install a 5 channel high speed disturbance analyzer to assist in determining the cause of the GFP operation.
15. NEVER remove the fuses or disable the GFP to prevent operation in order to restore power, you can and will be had liable for any deaths or damage.
16. If the cause of the GFP operation is a setting you believe is too low, ask the facility personnel for the coordination study to see what the engineer specified. Often the installing electricians leave the GFP set at the factory shipping settings which is the minimum for current and time. If a coordination study is not available (VERY TYPICAL) tell the facility representative they need to get their engineer of record to provide a setting. The normal response from engineers is what does the factory specify (NOTHING (well minimum) AS THE MANUFACTURE WANTS NO LIABILITY IN THIS ISSUE) or what do you normally set it at. My first response is I set it where the engineer tells me, then I give him my rationale for a minimum of 400 amps .1 sec. If they accept this I make them sign documentations stating it was the facilities decisions to adjust the relay settings.
1. Modern Switchboards have incorporated many optional protection schemes in addition to standard OCPs and GFP there is blown fuse protection, normally 3-KAZ fuses in parallel with the main fuses if the a main fuse blows the parallel KAZ blows and a spring actuator pops up to operate a microswitch resulting in operation of the Main Line Switch shunt. In addition the following or a combination of the following may be installed, phase relays, under voltage relays, and reverse phase relays to offer loads on the distribution system protection.
2. So prior to chasing a Ground Fault, one has to determine if the Switch operation was a result of a blown fuse, UV or phase loss. While the blown fuse operation is simple to find, it is obvious when you look at the KAZ and additionally some Switchboards have blown fuse indication lights on the front of the Switchboard. Phase loss and UV operation can be more difficult to diagnose, if the power is restored when you arrive on site.
3. Some GFP relays have indicators that will not allow you to close the Main Switch without resetting the indicator, others allow you to close the Main Switch with the indicator still showing a GFP operation and other GFPs systems have no indication of operation, leaving you to guess the reason the Main Switch opened.
4. In order of operation based upon personnel experience for Main Line Switch opening.
a. Electricians tracing circuits short a 408/277 VAC branch circuit, or electricians or others as part of construction short a 480/277 VAC circuit. Electricians that are still tracing circuits in this method are NOT electricians. *1 see below
b. Faulty equipment shorts to ground; number one here is motors, with cooling tower fan motors being number one, HVAC compressors number two. *2 see below
c. An actual ground fault with water leading the list for the cause of the ground fault.
d. A defective GFP relay or other protection device, defective CT or an open in the CT secondary wiring.
e. A fault in 208/120 VAC distribution in a properly installed system will not operate a 480/277 GFP
5. *1 In a typical office building the majority of branch circuit breakers are 20 amp and 30 amp, if the GFP relay is set at the 100, 200, or 300 amp setting a fault on a 20 or 30 amp branch circuit can result in an operation of the Main Line Switch. A standard molded case circuit breaker will operate in the instantaneous range at 6 to 10 times the rating of the CB 10X20=200 or 30X10=300, plus or minus 35% accuracy. So if the GFP relay is set low a majority of the branch circuits can result in a GFP operation. IMO this is not good safe coordination, I feel 400 amps should be a minimum (BUT I AM NOT AN ENGINEER).
6. *2 with motor faults and large equipment faults it is almost impossible to coordinate the GFP with the OCP for the utilization equipment. Assuming the same 6-10 times instantaneous rating of the OCP a 100 amp OCP would operate on a ground fault at around 1000-1200 amps which may be at or above the GFP relay settings (maximum setting 1200 amps).
HOW TO SAFELY RESET GFP PROTECTED MAIN LINE SWITCHES:
1. Try to determine if the Switch opened from GFP operation or other system protection device.
2. Ask if there is any on going construction.
3. Ask if the Switch opened at dusk, outside lighting can be an issue especially after periods of rain.
4. Check the GFP settings time and current, write them down.
5. Have someone (because it requires a lot of walking) shut off all downstream devices at the level below the Main.
6. Once you are sure the power is off, open and test the bus with a tester you are sure works (HAVE YOU TESTED YOUR TESTER). It is possible for Switches to hang up BECAREFUL.
7. Inspect the bus for any water and signs of a fault.
8. Megger the bus phase to phase and phase to ground, if the reading is low check that there are no relays connected to the bus, bus taps or other loads you are unaware of. Normally we test the bus at 100 VDC first to avoid damaging protective relays such as Phase or UV relays the retest at 1000 VDC. We?d like a reading above 50 megohms, 5 can be acceptable, .5 as recommended by some IS NOT ACCEPTABLE, IMO.
9. Open the next downstream OCP and then megger the feeders. Continue in this manner.
10. With HVAC equipment, motor loads, large duct heaters and other utilization equipment you will need to get to the load conductors for the equipment, do a visual on the equipment looking for signs of a fault.
11. Check for blown fuses and opened CBs as you work your way through the building.
12. Hopefully you will located the source of the fault, if you don?t the problem may be with the relay and require testing, and this is more than just pushing a button. A high current test set is required.
13. If you have done all of the above, with exception of item 12, you may have to close the Main Switch without resolving the issue, THIS IS CONTRAVERSAL and may violate local and federal rulings, BUT IMO is sometimes the only solution when all avenues have been exhausted.
a. Set the GFP relay to 100 amps (or lowest setting) and the time delay to the lowest time setting Instantaneous, .1 or minimum are typical minimum settings.
b. READ THE SWITCH OPERATION Instructions, the number of Service calls we receive for damaged Switches due to improper operation exceeds a level that would be obtained if Electricians READ THE INSTRUCTION.
c. With all downstream OCPs open, all covers installed on the Switchboard, all cover screws installed tight, you dressed in proper safety gear and no one else in the room; close the Main Switch in such a manner that places you in as safe a position as possible.
d. If the Switch holds and it should if you have done all the above steps as noted. Move on to the next level OCP.
e. Close these devices one at a time allowing any equipment that may need to start time to start.
f. If the GFP operates at any point and you gave equipment sufficient time to start you may have located the fault.
g. Reclose the Main Switch per step 13.c. and start closing all the OCPs leaving the one with the fault off.
h. Bring the building on line and return to the OCP with the suspected fault and isolate the source of the fault.
i. One issue to be aware of is; large dry type transformers have inrush currents that can trip GFPs relays due to the inrush current are not Zero Sequence.
j. Once you have isolated the fault, remember to reset the GFP settings to the ?AS Found Settings?, reset the GFP indicator (if present).
14. If after this you have not located any problems, the GFP relay should be left set at the lower settings, the GFP tested at all presets for current, and if all checks out it may be necessary to install a 5 channel high speed disturbance analyzer to assist in determining the cause of the GFP operation.
15. NEVER remove the fuses or disable the GFP to prevent operation in order to restore power, you can and will be had liable for any deaths or damage.
16. If the cause of the GFP operation is a setting you believe is too low, ask the facility personnel for the coordination study to see what the engineer specified. Often the installing electricians leave the GFP set at the factory shipping settings which is the minimum for current and time. If a coordination study is not available (VERY TYPICAL) tell the facility representative they need to get their engineer of record to provide a setting. The normal response from engineers is what does the factory specify (NOTHING (well minimum) AS THE MANUFACTURE WANTS NO LIABILITY IN THIS ISSUE) or what do you normally set it at. My first response is I set it where the engineer tells me, then I give him my rationale for a minimum of 400 amps .1 sec. If they accept this I make them sign documentations stating it was the facilities decisions to adjust the relay settings.
