Electrical Troubleshooting Quiz — August 2, 2022

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An 80-person manufacturing plant had problems with a particular production machine. For example, they have already replaced nine circuit boards there this year. The factory manager called the manufacturer and the support technician said he had never encountered such a problem before. However, he said it sounded like voltage transients were removing the circuit boards. He suggested finding the source of the intermittent peaks and sags, correcting the underlying problem, and then seeing if the part failure rate starts to drop.

The plant manager called your company and your boss assigned you to the project. What are the troubleshooting items for this issue?

Ground connections

The support technician may have identified a problem, but their solution may not be practical because factories are full of transient voltage sources. For example, every engine that starts and stops on the line will contribute to this problem. Still, the first step in troubleshooting is to characterize the power supply. Put a power analyzer on the charger at that particular load. If you can secure it against theft, leave it connected to collect data for a few days. If you can connect another to the branch circuit(s) for this equipment, that may provide more information.

Installation errors are more likely here. Why? Because circuit boards are on the load side of a DC power supply. In addition to a transformer, the power supply must have a Zener bridge or other voltage regulation scheme, as well as capacitors and filter coils (tank circuit). It probably also has its own transient protection circuit. The power supply should provide more than adequate protection against normal electrical noise.

However, unwanted current can enter the electronics by finding another way than through the power supply. Look at each chassis where the circuit boards are mounted. You may see several terminals or connection points labeled with an identifier that includes the word “earth”. For example, there may be a chassis ground, a power ground, a signal ground, and a common (or just common) ground, but none of them are actually grounded. Art. 100 defines ground as a connection to ground.

Start by determining if neutral and ground are interconnected anywhere. This is a common error, but correcting it can involve extensive wiring. You will need to estimate the repair time. Proceed methodically.

Next, look at the mass of the chassis. The chassis is a non-conductive metallic body, which means that it must be at the same potential as other important non-conductive metallic bodies. Otherwise, you get potential differences. If the chassis is connected to ground rather than an equipment bonding conductor, it is not equipotential to other metal objects and therefore unwanted current flows.

It wouldn’t take much of that current to zap a circuit board. This is why computer technicians wear crocodile straps attached to the computer chassis; their hand is then at the same potential as the chassis, the chassis is at the same potential as anything plugged into it, and the technician’s hand will subsequently introduce no current that zaps a motherboard or expansion card .

And it could be that the factory’s own technicians are inadvertently zapping these cards due to poor electrostatic discharge practices. Ask to see preventative maintenance procedures and if a service technician can perform one or a dry run as you observe.

If any part of this equipment is actually connected to ground (e.g. someone drove a ground rod) or connected to building steel as the only return path or bonding path, you have a big red flag indicating that all equipment ground connections are not correct. Identify gaps and estimate the time needed to correct them.

You will need to review your power analyzer(s) information. If you have a high-end model, you can let it convert raw data into useful information. If it shows that large transients have appeared on the power supply or branch circuit, the source is probably in the factory. And it would probably be a big engine starting on the other side of the line. Typical culprits include fire pumps, factory air compressor motors, and stamping press motors. An overloaded scrap crusher can also be a contributor.

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