Frame Leakage Protection
- Thread starter Diver1
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rcwilson
Senior Member
- Location
- Redmond, WA
I believe frame leakage protection is a method of detecting a ground fault in equipment by isolating the enclosure from earth (ground) and making a single connection to ground through a current tansformer. Any fault in the equipment or any ground "leakage current" from the energized parts to the equipment frame or enclosure causes a current through the CT and activates the ground fault relay.
This protection is IEEE standard number 64 or 64G. I have only seen it applied on old (1950's) transformers and large rectifier enclsoures. Some utilities put a CT around the support post for high voltage instrument transformers or live tank circuit breakers. The CT can measure deterioration or surface leakage across or through the procelain insulator.
Note that this method does not violate code because the equipment is bonded to ground through the CT. However, it is difficult to maintain that single point enclosure grounding. Any conduit connection, anchor bolt, or installation brace fastened to the enclosure shorts out the protection.
This protection is IEEE standard number 64 or 64G. I have only seen it applied on old (1950's) transformers and large rectifier enclsoures. Some utilities put a CT around the support post for high voltage instrument transformers or live tank circuit breakers. The CT can measure deterioration or surface leakage across or through the procelain insulator.
Note that this method does not violate code because the equipment is bonded to ground through the CT. However, it is difficult to maintain that single point enclosure grounding. Any conduit connection, anchor bolt, or installation brace fastened to the enclosure shorts out the protection.
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
I have seen this done with a control panel that was mounted inside switchgear. The panel was mounted on insulators & a single wire taken to the ground bus. The wire was routed through a CT to detect current flow.
skeshesh
Senior Member
- Location
- Los Angeles, Ca
I've never used this type of protection but I'm interested; what are you trying to accomplish?
rcwilson
Senior Member
- Location
- Redmond, WA
Fast, sensitive ground fault protection on high power/current equipment.
For example, we had transformer/rectifier enclosures rated 1000VDC, 20kA DC output, fed at 13.8 kV. The 13.8 kV phase CT's for the overcurrent relays were 4,000:5 with a derived neutral (50N) relay. The ground fault pick up setting had to be set at 1,000 Amps to avoid spurious trips. The rectifier transformer's low voltage windings feeding the diodes were protected by the diodes' high current fuses. Again, their rating was so high they would not protect much for a ground fault in the equipment.
All of the internal supports for bus, fuses rectifiers and other energized components were mounted on a steel frame that was insulated from ground but bonded by a bonding jumper (busbar) through a CT. The CT fed a sensitive ground fault relay set around 100 A AC with enough time delay to ride through start up inrush charging currents. A DC shunt and sensitive DC overcurrent relay protected for faults on the DC side of the equipment.
The 64G protection allows sensitive settings because no current should be flowing during normal operation.
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