5 Pitfalls to Avoid While Testing Underground DC Battery Systems

A battery system provides standby and emergency power to operate industrial consumer, commercial or protective devices, like emergency lighting units, uninterruptible power supplies, continuous process systems, operating controls, switchgear components, and protective relays. In emergency situations, it is crucial that these devices work properly. Failure of a DC system or its battery can result in operational failure of devices connected to that system. If this happens, loss of revenue, damage to equipment and/or injury to personnel may occur. This is why it is important to be aware of the potential of ground faults.

What is a ground fault

A ground fault is inadvertent contact between an energized conductor and ground or a grounded piece of equipment. The return path of the fault current goes through the grounding system and any personnel or equipment that becomes part of (is in contact with) that system.

Oftentimes a floating DC system can develop grounds within itself. When a battery system is partially or completely grounded, a short circuit is formed across the battery and may cause the protective device to fail to operate when needed. Some of these causes include leaking cells, corrosion, dust, dirt, pinched wiring, welding sparks, pest damage, poor or aged insulation, or faulty component.

Identifying a ground fault on a battery system

Utilities and industrial complexes go through a great deal to find ground faults within their battery systems. Locating these faults is no easy task. They can prove to be elusive, creating a time-consuming process. Ground fault tracing is both part science and part ‘art’. But by using the proper tool, much of the ‘art’ is taken out of the tracing, reducing the time to find the fault.

Finding a fault depends on many things, including the size of the system and how spread out it is over a plant or facility. It is also dependent on access to areas, panels, and junction boxes. Knowledge of how the cable is routed in a facility or plant is a plus. Tools also play a critical role in locating and troubleshooting the fault. Using a good ground fault tracer and having system drawings are important. Hand tools, PPE, a notepad, and any background information available, like any recent installation or maintenance activities performed on a system or if it has rained recently are also key in helping to identify where a ground fault is in a system.

Ground fault tracing seems simple, right? A transmitter applies a signal, the signal flows through the ground fault, and the receiver traces the signal. But it’s not.

Identifying circuits with a signal from the transmitter can be tricky. There could be interference on the DC system and the signal can split amongst multiple paths. Which one do you trace? Which is the real fault, and which is a phantom fault? There could also be a weak signal due to a high impedance fault. This could cause high leakage capacitance in the circuit.  Accidentally tripping a circuit when working in a cabinet is also a possibility. Especially if there are loose wires. Knowing how to avoid the pitfalls of ground fault location on a battery system can help you when looking for the ground fault.

Five pitfalls to avoid when looking for ground faults

• Leaving a ground fault monitor on – Leaving a monitor active may cause alarms and communicate with the system controller, giving you an incorrect reading when looking for a ground fault. Be sure to disable the ground fault monitor prior to trying to locate any faults on your system.
• Stray capacitance – Capacitance acts as a variable impedance. It impacts AC and DC signal. A pulsed DC signal causes a charging effect on the capacitor. Pulse energy is stored in the capacitance. The pulse amplitude is dampened, making tracing difficult. A way to fix this is to increase pulse amplitude or direction as well as mind the amplitude to avoid inadvertent breaker trips. Circuits with high capacitance also draw current from the test instrument making it difficult to distinguish between an actual fault and a phantom fault. Using a capacitive pickup to determine phase shift allows for the real and reactive components of the current. Circuits with only resistive current are examined.
• Inadvertent breaker tripping – Ground fault current may flow through a breaker tripping circuit. Low impedance draws higher current, making a breaker trip. High impedance can occur when over voltage protection may operate, causing a breaker to trip. To combat this, troubleshoot from the DC system where the fault is located. Identify which side the fault is located on. This helps to avoid forcing the tracing signal to flow to the other side. It is also important to remember that you should limit the parameters of the tracing signal, especially since you don’t need a lot of signal to trace. For pulse DC that would be amplitude and duration. For AC that would be voltage and current.
• Potential noise on system – Noise on a system can look like a pulse in a tracing tool. If the noise magnitude is high, even during the pulse and detection, the noise can still interfere. Pulsed output needs synchronization with a receiver.
• CT Magnetization – Pure iron core CTs provide high permeability. Load and charging DC current may magnetize. It can be very difficult to remove the CT from the circuit. The best option is to use a CT with nickel alloy core. It doesn’t magnetize easily and provides high permeability.

Following the steps to avoid pitfalls and using the right tracing instrument can bring reliability and effectiveness into the troubleshooting process. Megger’s MGFL100 can help you get the job done. It only alarms when the real fault is identified. It is able to take a direct measurement of resistance and stray capacitance, as well as direct measurement of fault current and leakage current. It also offers adjustable current and voltage limits. All to help you keep the power on.