DC systems can provide reliable DC operating power for various control devices, automatic devices, relay protection, and signal systems, and also serve as reliable operating power supplies and emergency backup power supplies. When the insulation level between the positive pole or negative pole of a DC system and the ground drops to a certain setting value, it is generally referred to as DC grounding. When the insulation level of the positive/negative pole is lower than a specified value, it is called positive/negative grounding, which may cause maloperation of protection devices. For a DC system with a single-point ground fault, the power supply reliability is greatly reduced. It is imperative to locate the grounding point and eliminate the fault as soon as possible to avoid potential hazards.

Two-point grounding may cause circuit breaker maloperation: Grounding at points A & B, A & C, A & D, or D & F may lead to circuit breaker tripping by mistake.

Two-point grounding may cause circuit breaker refusal to operate: Grounding at points B & E, D & E, or C & E may result in circuit breaker failure to trip.

Two-point grounding may cause fuse blowing: Grounding at points A & E may trigger fuse melting.

When grounding occurs at points B & E or C & E, protection devices will actuate but circuit breakers will refuse to operate, fuses will blow, and relay contacts may even be damaged.

The HBD-81 DC Ground Fault Locator consists of three components: a system analyzer, a branch detector, and a data collector. It adopts high-precision current clamp meters and locates faults by detecting the DC current difference in fault circuits. The fast FFT transform technology is integrated into the device, enabling it to detect various insulation faults, DC cross-circuit faults, and AC intrusion faults in DC systems of all voltage classes.

△ HBD-81 DC Ground Fault Locator

Testing Method

1. Deactivate the original on-line insulation monitoring device of the system.(The original device is equipped with a balancing bridge and a detection bridge, which will conflict with those of the HBD-81 analyzer. If not deactivated, the insulation resistance calculated by the analyzer will be inaccurate.)
2. Connect the positive, negative, and ground terminals of the analyzer to the system under test. The analyzer is usually connected to the busbar. Turn on the power switch; the analyzer will detect the system status and calculate the insulation resistance between the positive/negative poles and the ground. If the insulation resistance of either pole to the ground is less than 999.99KΩ, a fault will be displayed; otherwise, the system will be shown as normal.

3. After the analyzer completes the test, if an insulation fault is detected in the system, use the insulation analysis function of the detector to test the ground wire of the analyzer. If the detector displays a two-cycle square wave, it indicates that both the analyzer and the detector are functioning properly, and the detector can be used to test each feeder line.
4. During detection，When clamping near the busbar side of the fault point, the detector will display the fault current waveform.When clamping beyond the fault point (i.e., near the load side of the fault point), no fault current waveform will be displayed. This enables the accurate location of the fault point.

5. If a clear fault current waveform is detected in the circuit under test, an insulation fault is confirmed. After the detection, the detector will display an upward or downward arrow to indicate the relative position of the fault point.An upward arrow means the fault point is in the direction marked by the clamp meter.A downward arrow means the fault point is opposite to the direction marked by the clamp meter.For personnel familiar with the circuit layout, the arrow indication can be ignored: if a fault current waveform is detected at the current clamping position, an insulation fault must exist on the load side of the line.

6. Wireless communication is adopted between the detector and the analyzer. After successful communication, the detector will store the communication data as long as its power switch remains on. Real-time communication is not required during subsequent tests, allowing the detector to be used at other positions of the DC system without moving the analyzer.

Precautions

1. The detection bridge of the analyzer has two modes: Auto Mode and Forced Mode.In Auto Mode, if the external resistance changes significantly, the analyzer will exit the current detection bridge state, recalculate the system-to-ground resistance, and decide whether to activate the detection bridge based on the system condition.In Forced Mode, the analyzer will maintain the current state of the detection bridge regardless of changes in external resistance.
2. When testing at the back of the main feeder panel, clamp both the positive and negative feeder lines together as much as possible.The ground leakage current to be detected is generally at the milliampere level, while the load current of feeder lines from the main panel is usually at the ampere level. Clamping a single line may exceed the detector’s measurement range (1A) or introduce heavy interference from the load current, affecting fault judgment. For rear-stage protection panels or terminal boxes with low or no load, single-line detection is acceptable. If the detector displays "clamp saturation" during testing, it indicates an excessive current in the circuit under test, and both positive and negative lines must be clamped together.