Out of sight, out of mind is a common aphorism, but when it’s applied to standby backup batteries, it can — and does — become catastrophic. Standby battery banks support production lines, hospitals, computer rooms, and entire facilities to provide an alternate power source should utility power fail. The standby battery is the interface between a smooth transition and total catastrophe, yet it is often in a basement, a locked room, or even a separate building. With no moving parts and seemingly doing nothing, it is easily overlooked and forgotten…until called on-line.
Maintenance is required on these batteries just as on visibly active equipment. They are visibly static, but electro-chemical activity is occurring. Corrosion, dry-out, spontaneous discharge, and other unwanted effects can render the battery bank inoperative when it’s most needed.
MAINTENANCE
Because a severe threat is posed by electrical leakage to ground, which can drain the battery of charge, a maintenance program should include detection, location, and elimination of all such unwanted leakage paths. These paths can be varied, diverse, and difficult to fully detect.
Moreover, the use of generic electrical test equipment, like digital multimeters (DMMs), is far from sufficient and cannot effectively identify and trace the many variable possibilities. Specialized equipment, designed to address these variables, is in order. Steady advancement has been made in developing ever-more precise and diverse tracing techniques, and this article brings the reader up to date on the most recent advancements.
Standby battery banks can spontaneously develop unwanted paths to ground through damaged wiring, faulty insulation, dirt, and dust. Such faults create hazards to personnel and equipment. Low-resistance faults provide high current, while high-resistance faults produce high voltages through differences of potential. Serious safety issues including fires, and shock potentials are the result. North America is under North American Electric Reliability Corporation (NERC) regulation for protection against these dangers.
Ground faults
For clarity, a distinction should be drawn between leakage and ground fault. Electrical leakage is typically a small, unintended current flow that can be acceptable at times. A ground fault is a more significant current flow to ground that needs to be corrected. Numerous common hazards contribute to this danger:
Pinched wiring is a typical maintenance error.
Pests such as rodents and insects contribute to wiring damage by gnawing on insulation and other activities.
Water ingress is a common hazard that can cause ground faults to develop suddenly as the result of a storm or through gradual erosion by moisture.
A single fault will not trip out vital equipment for ungrounded or IT grounded systems, but a second ground fault on the same circuit can cause high levels of current flow and lead to catastrophic failure (Figure 1).
Such circuits are protected by ground fault monitors, but these only indicate when a ground fault occurs. They do not tell the operator where the fault is actually located.
The basics of fault tracing are simple. A transmitter applies a distinctive signal through the ground fault and a receiver traces it (Figure 2). However, while this is fundamentally simple in concept, complications in actual practice include high leakage capacitance in the circuit, noise, a high-impedance fault, and tripping of breakers.
Capacitance. All cables have capacitance (Figure 3). This is the product of two conductors separated by insulation. The longer the run, the greater the capacitance.
To a common tracer, this can be mistaken for a false path to ground. You’ll trace all day and get nowhere. But modern technology comes to the rescue. Sophisticated high-tech tracers indicate real current and reactive current drawn by stray capacitance and can distinguish between them.
Noise. Noise, such as from a SCADA system, can be intense enough to swamp out the trace signal from a common tracer. An advanced, full-technology fault locator will output a low-frequency signal, typically 5.12 Hz. The receiver’s low pass filter measures the low-frequency signal from the transmitter at a value well below and distinct from that of noise.
High Impedance Fault. High-impedance faults result from water ingress into the cable’s insulation. Water provides a path for electrical leakage to ground, but the surrounding insulating material makes it a high-impedance fault that creates a need for enough test current to trace it. Careful current regulation is in order lest the fault dry out before it is recognized. Sophisticated testers can trace faults up to 40 kΩ with minimal energy that allows tracing to proceed long enough to fully trace the fault. Less sophisticated instruments may require 1,000 V, which can dry out the fault before tracing can be completed and can be dangerous to the operator, trip breakers, and drive too much current.
Tripping. Breakers must be protected from tripping that can take critical equipment and systems off-line. Applying too much current while trying to trace a fault can cause tripping and must not be allowed. A sophisticated tester will prevent this by providing adjustable limits on maximum current as well as maximum voltage outputs. These in turn can be password-protected to prevent being inadvertently and possibly destructively changed.
TRACING THE FAULT
Standard operation begins with connecting a transmitter’s three output leads, two across the battery string and one to ground. The transmitter will indicate which side of the string has the fault. The faulted side is then energized. Typical values are 10 V or 50 mA. The transmitter will indicate the impedance and capacitance of the tested circuit. A receiver is then connected to the transmitter’s output. With a fully featured tester, the receiver will indicate the current of the real fault and that drawn by stray leakage capacitance. A limit trigger can then be set. The receiver’s CT is applied to each circuit on the panel and visual notification and an audible beep will indicate the circuit with the fault.
Tracing the faulted circuit can provide challenges such as splitting into multiple paths. Which are real and which are due to stray capacitance? The receiver can be equipped with a specialized pickup that can distinguish between a genuinely faulted circuit and mere leakage.
Following circuits can lead the technician into barely accessible tight spaces that can physically defeat the whole project. However, a pickup technology known as a high-sensitivity flux gate allows a sensor to utilize a small core and a slide opening that can function effectively in the tightest areas.
Magnetization is a common CT problem that can be overcome with advanced technology. If a direct current (DC) is flowing and the line is clamped with a CT, the CT can become magnetized and be difficult or impossible to remove while being damaged in the attempt. A more sophisticated CT will have a Mu Metal core instead of iron. This material does not strongly magnetize and the CT will not become locked on the line.
On typical TT grounded circuits, a fault will trip a breaker. Some critical circuits cannot afford to go off-line. If an IT ground is used, it will not trip off-line in the event of a ground fault. Such faults can be located on IT grounded systems with the use of a specialized filter that can be placed between the transmitter and the circuit and will permit tracing AC ground faults up to 600 V. Similarly, a full-featured receiver can be placed in 50/60 Hz mode and trace fundamental currents on any type of line.
CONCLUSION
Many potential paths to ground can drain a battery of standby charge and render it inoperative at the most crucial time. Including a state-of-the-art ground fault tracer in the maintenance program can prevent catastrophic failure that can result when a backup battery system goes dead.
Jeffrey R. Jowett is a Senior Applications Engineer for Megger in Valley Forge, Pennsylvania, serving the manufacturing lines of Biddle, Megger, and Multi-Amp for electrical test and measurement instrumentation. He holds a BS in biology and chemistry from Ursinus College. Jowett was employed for 22 years with James G. Biddle Co., which became Biddle Instruments and is now Megger.