Ground Fault Tracers, the BGFT and BGL

Andrew Sagl - Product Marketing Engineer

There have been many people with the same basic question, “Which ground fault tracer should I buy, the BGL or the BGFT?”

There are several main differences between the two instruments including relative ease of use, output power and line-versus battery-operated. The similarities are that they both find (and measure) very well single and multiple faults in ungrounded DC systems. They both do their jobs while the battery is on-line and neither one trips circuit breakers because they both ramp the current and voltage. The differences are dicussed in detail below.

Both utilise the same basic principle: they both inject a low frequency a.c. signal across one side of the d.c. bus to the station ground creating a return path for the signal. The signal will flow through the d.c. bus to the fault location and then pass to ground. The connection to the station ground makes a complete circuit enabling tracing the fault to its exact location. Simply trace the injected signal. Any branch circuit with a fault will have some amount of injected current. Any branch circuit without a fault will have no a.c. signal.

In generating stations with many rotating machines and high electrical noise, the BGFT is the only one that should be recommended because of its 80 W of output power (compared to 3 W for the BGL).

In substations, noise is usually not as critical a criterion as other aspects such as user knowledge level, frequency of faults and ease of use. To be perfectly honest, ease of use is a relative term depending upon how each person defines it. the process of tracing faults should be an infrequent event and therefore, the instrument and its use must be easy enough that a complete training programme is not required for each fault-finding event.

Both instruments can trace faults in “normal ranges of resistance” that most people worry about. The BGFT can detect resistance faults up to about 400 kO and the BGL can detect resistance faults up to about 100 kO. In many cases and depending upon individual companies’ protocols, faults are not traced until the resistance falls below 25 to 50 kO range which is usually when a second fault occurs. Then both faults are traced and corrected. The one exception to this, generally, in the nuclear industry where the first fault is traced immediately.

Both the BGFT and the BGL can be very easy too use. Both units have bridges to measure capacitance and resistance, individually. The BGL has an automatic bridge which is easier than the BGFT’s manual bridge. Of course, the manual bridge need not be used. However, if the bridge is not used, it is possible to trace phantom faults due to high capacitance.

The bridge, whether manual or automatic, is fused to search for the circuits with the real faults, i.e., resistance versus capacitance. Using the manual bridge of the BFGT can be time consuming in one respect; verifying each branch circuit with a high current to determine which branch(es) has the fault(s). In the other respect, tracing phantom faults can also be time consuming and there is more walking involved. If, however, there have been many faults and it is a well-charactised system, usually the culprit circuit is known and simply needs confirmation. A bridge is not required in this instance.

The benefit of having a stationary, line-powered transmitter connected at the battery or first panel is that the “receiver-CT” which is used to take the readings of total current (resistance plus capacitance) at each branch circuit is carried to each panel. The Receiver-CT weighs less than one 1lb (1/2 kg). The downside to the Receiver-CT is that there is no capability to separately measure resistance from capacitance. Simply trace any high reading and usually there aren’t so many of them. Therefore, tracing phantom faults is not very time consuming and the readings decrease linearly in value as one goes further into the d.c. network.

The BGL combines the BGFT’s Transmitter and Receiver-CT in a single unit. The advantage is that only real faults are traced using the automatic bridge at each panel and each branch circuit. There are no extra knobs or adjustments, simply push a switch and measure. It is fully automatic. The BGL is battery operated and is relatively lightweight at 25 lbs (11 kg).

Some helpful hints to aid in quickly finding faults: 

• Mark each branch circuit with the base value
•  Know where the problem areas have been (during rainy seasons, sensitive components, etc.)
•  Have schematics readily available

The differences are not so great as to cause major concern and both work very well. Both have been in production for many years are well supported and very reliable. It is hard to make a mistake with either one (with the exception of noisy systems). The real mistake is not taking advantage of the technology. Turning off breakers to find a fault is both dangerous and time consuming, not to mention unnecessary. The BGFT and BGL are the answers to many a maintenance person’s ground fault nightmares due to their ease of use and simplicity.

Please note the BGL is not CE marked