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Locating Battery Ground Faults

 

Ground faults often develop in substation battery systems over time due to environmental factors and maintenance activities. Dirt and debris accumulated on battery surfaces can create electrical paths to the battery rack. Electrolyte residue due to cracking or crazing of the battery container also results in conduction paths to the battery rack. This application note will cover how to use the Megger BGL to trace and locate ground faults on a live DC system, without sectionalizing any circuits.

PREPARATION
Before starting the test, make sure that you have:
• Personal Protection Equipment 
• DC system diagrams
• Access to each circuit breaker panel board

STEP 1: Turn on the BGL


The BGL can be powered directly from the mains or from its internal battery. After turning on the unit, the “In Progress” LED will be illuminated. If the instrument is running on the internal battery, the “Overrange” LED will also be illuminated when the charge is over 90%. 

STEP 2: Connect test leads 


Connect the current sensor and output lead set to the instrument. The output test lead set should be connected to the battery system in the following order:

1. Connect both the BLACK and GREEN clips to ground.
2. Connect the RED clip to the positive or negative battery bus bar. A small spark may occur when connecting to battery terminals and the “Warning > 30VDC” LED may turn on.
3. Clamp the current sensor around the RED cable from the output lead set.

STEP 3: Measure total resistance to ground 


After connecting the clamp, set the “Function” switch to resistance position and wait for the “Valid” LED to illuminate. Verify that you can measure the ground fault and note the value.
With the current sensor around the red cable of the lead set, the instrument will measure the total parallel resistance to ground of the battery system. This includes all leakage paths from both positive and negative sides. 

If this resistance is in excess of 10 kΩ, it might indicate the presence of multiple high resistance faults which the BGL is not equipped to handle. A better instrument to use would be the Megger BGFT.

STEP 4: Measure resistance of each branch


If the total resistance measured in Step 3 is below 10 kΩ, the next step is to measure the resistance of each branch of the system to track down individual faults. 

To measure the resistance of a branch, disconnect the clamp from the red cable of the output lead set and clamp it to the branch to be measured. Wait for the “Valid” LED to illuminate and note the resistance to ground of this branch. Repeat this for all branches. 

STEP 5: Locate specific fault points


Further measurements should be done downstream along each branch to locate the specific point of fault in the branch. Compare the resistances recorded in Step 4 and start tracing from the lowest resistance branch to the highest.

STEP 6: Disconnect test leads


Once all tests are complete, remove the test leads in the following sequence: 
1. Remove the clamp from the branch.
2. Disconnect the RED clip from battery bus bar. If the “Warning > 30VDC” LED illuminates, connect the RED clip to ground until the light turns off. 
3. Disconnect the BLACK and GREEN clips from ground.
4. Turn off the unit and disconnect it from the power supply.
About the BGL
The BGL was designed to detect, track and locate ground faults on both live and offline battery systems without sectionalizing. The BGL simplifies fault tracing by differentiating between resistive fault currents and capacitive charging currents. This allows the instrument to operate in high electrical noise environments, such as in the presence of surge-suppression capacitors. 

 

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