Gas-insulated substations (GIS) have transformed how high-voltage infrastructure is built and maintained. By encapsulating conductors, circuit breakers, disconnectors, and instrument transformers inside grounded metal enclosures filled with SFâ gas, GIS designs dramatically reduce footprint and environmental exposure. But this very design creates a significant testing challenge: the main circuit terminals of a GIS circuit breaker are not directly accessible.
For decades, timing and contact resistance measurements required maintenance crews to disconnect the ground connection from at least one side of the breaker. This eliminated the parallel current path through the enclosure and grounding system, but it also introduced safety risks, extended outage windows, and complicated what should be a routine maintenance task.
DV Power's application note outlines a modern alternative: testing with both sides grounded (BSG) using the CAT500 and the GIS Test Module.
Why Conventional Methods Fall Short in GIS
In a conventional substation, test leads clip directly onto breaker terminals. In a GIS, the breaker interrupter is buried inside a sealed, grounded enclosure. The only access points are typically detachable shunts—maintenance switch terminals that bridge the isolated main contacts to the grounded enclosure.
If you attempt a standard timing or micro-ohm test with both sides grounded, the test current splits between the main contacts and the parallel grounding path. The result is unreliable timing triggers and contact resistance values that reflect the parallel combination, not the true main circuit resistance.
Historically, the workaround was to remove one side's ground connection. That opens the parallel path, but it also means:
- Additional switching operations and isolation steps
- Longer exposure of personnel to high-voltage compartments
- More complex safety documentation and clearance procedures
- Increased risk of accidental re-energization or induced voltage exposure
The BSG Method: How It Works
DV Power's BSG method is built around two core principles: injecting high DC current through the parallel network, and measuring the current response in a way that allows the instrument to mathematically separate the main circuit from the ground path.
The hardware setup uses the CAT500 control and analysis unit paired with the GIS Test Module, which contains three battery-based current sources capable of delivering up to 400 A DC each.
For timing measurements, the test current is injected through the primary circuit via the test terminals (detachable shunts). The response is captured not by monitoring voltage drop, but by recording the current signal on the secondary side of the current transformers using AC current probes connected to the GIS Test Module's analog channels. When the breaker operates, the current interruption in the primary circuit is reflected as a current change in the CT secondary—providing a precise timing trigger without ever lifting a ground connection.
For contact resistance, the approach is equally elegant. The system performs two measurements automatically:
- Open-circuit resistance (Rgr): Measures the resistance of the grounding path and GIS enclosure alone.
- Closed-circuit resistance (Rp): Measures the resistance of the parallel combination of the main circuit and grounding path.
The CAT500 then calculates the true main circuit resistance (Rc) using the formula:
Rc = (Rgr × Rp) / (Rgr − Rp)
This eliminates the need to isolate the ground path manually while still delivering an accurate micro-ohm result.
Connection and Configuration
Physical connection is straightforward but requires attention to detail. Three sets of current cables run from the GIS Test Module to the test terminals on both sides of the breaker. AC current probes clip onto the CT secondary wires—either at the GIS enclosure itself or in the breaker control cabinet. DV Power recommends using the CT's measuring core when available, as it provides the strongest signal response.
For breakers without accessible test terminals, clamps can be attached to the nearest conducting points on the enclosure adjacent to the maintenance grounding switches.
In the CAT500 software interface, enabling the BSG feature is a matter of selecting "Use modules: Yes" and choosing "GIS" on the starting screen. From there, the operator selects Timing/GIS or Static Resistance, configures the breaker type as GIS, and chooses the appropriate test method:
- Contact current measurement: DC clamps positioned close to the contact terminals to measure current flowing directly through the main circuit.
- Earthing loop current: DC clamps capture current in the grounding loop; the CAT500 calculates the contact current by difference.
- One side grounded: Conventional Kelvin method for situations where BSG is not required.
The current probe ratio is set according to the clamp range (10 mV/A for 200 A, 1 mV/A for 2000 A), and the operator can test individual phases or all three simultaneously.
Practical Considerations
A few details from the field are worth keeping in mind. The AC current probe method for timing relies on a CT turns ratio of 1000:1 or lower. Higher ratios may produce a signal too weak for reliable detection. In those cases, the alternative is to place adaptable AC current probes directly on the test terminals to record the primary circuit response.
For three-pole-controlled breakers, the standard BSG resistance measurement yields the combined resistance of all three phases. If per-phase values are required, DC current probes must be connected to each phase's test terminals so the instrument can resolve individual current paths.
The Bottom Line
The BSG method represents a meaningful shift in GIS maintenance philosophy. Instead of working around the grounded enclosure as an obstacle, the test system treats the entire circuit—main contacts and ground path—as a measurable network and extracts the value of interest through intelligent instrumentation.
For asset owners and service teams, the benefits are immediate: reduced switching operations, shorter outage durations, simpler safety protocols, and less physical handling of GIS compartments. The CAT500 and GIS Test Module turn a historically cumbersome procedure into a safer, more efficient standard practice.
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