Ground Fault Monitoring in Three Phase Systemsby Ted Smith on 10/18/17
Ground Fault Monitoring in Grounded AC Systems
Most electricians are very familiar with a current transformer based ground fault current relay due to their understanding of how a GFCI circuit breaker or receptacle works. Even non technical personnel encounter them on a daily basis in public rest rooms protecting a wall outlet in a wet area. Ground Fault Protection, sometimes also called GFI protection, works the same way except it is for services. The purpose of Ground Fault Protection is to monitor services for faults to ground and open the main breaker. Ground Fault Circuit Interuption in 120 volt circuits at receptacles and circuit breakers differs from Ground Fault Protection in one key way. GFCI protection is primarily designed to protect personnel from electric shock. Ground Fault Protection is not designed to protect people from electric shock, instead it is designed to protect the system and equipment from the effects of ground faults. Although the two types of protection differ in their purpose, they work in with same operating theory.
The operating theory behind the relay is as follows. A current transformer (CT) or “donut” is placed around the power wires leading to the protected load. It is important that hot and neutral wires are fed through the CT. This goes for both, single phase and three phase systems. One might come across a three phase system without a neutral, feeding a pump or an industrial motor. In this case the three phases only will be fed through the CT. Basic rule for three phase systems: If the neutral is being carried out to the load, feed it through the CT. If the neutral is not being used, then it may be left as is.
The current transformer will always read zero current in a healthy system even under a full load condition. In accordance to Kirchhoffs laws, Incoming and Outgoing currents will cancel each other out. Assume a 10A load connected to a 480/277VAC system. 10A will be fed from the source into the load, therefore 10A will have to return from the load back to the source. The CT will measure both simultaneously since it is placed around all conductors. The values were randomly chosen. Below is what the CT would see at a specific moment in time:
In accordance with the attached schematic: 10A - 5A -5A= 0A for a healthy system
Figure 1 Attached and Named Guideline 7: The grounded system with single ground fault and GF relay
A ground fault (for this example, 1 A) will divert some of the current from the arrangement and bypass the CT via the ground wire, a frame or the building ground and return back to the source.
The new equation for the CT is now: 10A - 5A - 4A = 1A; 10 A go into the load, 9 A return to the source via the phase L2 and L3 and 1 A returns to the source via the ground wire. The CT will step the current (1A) down and forward it to the Ground Fault Relay (GFR). The GFR will then alarm when its set point has been increased. The GF relay in combination with a zero sequence CT will work in resistance grounded systems as well. It will run into its limitations in circuits where wave form modifying equipment, such as Variable Frequency Drives (VFDs) or rectifier components are installed. A more advanced technique is employed for DC circuits as well as circuits with variable frequency drives.
A critical change to the 2017 NEC now requires that the GFP for services must be tested at the time of installation. This has been a requirement for some time now but the new change limits the type of test to a current injection test only now. This means that we will have to plan in advance because current injection testing can not be done if the line side feeders are terminated. We will have arrange for the testing, have it completed and do our feeder terminations.