Grounding and Bonding

Maintaining good bonding on traffic signal installations is an important part of the maintenance of these systems. Most traffic signal standards, controller cabinets, and etc are metallic and therefore are good conductors. When these systems are properly installed, they are bonded together and grounded to provide protection from shorts of the wiring to the conduits, standards and cabinets. If there is a wiring fault, the circuit breaker will then trip.

If the bonding is not installed, is broken, or has a high resistance then, not only might the circuit breaker not trip if there is a short but the conduit, standard or cabinet my become hot and present a shock hazard.Visual inspection may not be enough. A high resistance connection may occur while the mechanical connection still exists. This is especially true in older installations. A Ground Resistance Tester should be used to identify problems. Of course, you could also just measure for voltage across connections and between conduits and standards or to the neutral conductor.

I remember two occasions in particular where this problem exhibited itself.

A bicyclist called and said he felt a tingle when he pushed the bike push button. When I checked the location, I didn’t feel the tingle, but again, that doesn’t mean there isn’t a problem. I measured 35 volts AC from the standard to the conduit. My electrician shoes do work!

Since I measured the voltage, it means there were at least two problems with the installation. Of course, the bonding had a problem. But, also rather important was why was there voltage present at all. Was it just inductive or was there a wiring fault. Of course there was a fault. Induced voltage in typical systems with typical wiring distances is usually below 5 volts. I inspected each traffic signal head and found a pinched wire from one of the lamp sockets to the reflector frame. I corrected the fault then corrected the conduit bonding. Then checked the voltage on the standard and all was well. I then checked all the other standards.

On another occasion, I was troubleshooting a traffic signal wiring problem looking for why a conflict monitor was randomly tripping. The induced voltages at the cabinet field terminals were higher than normal. When I measured in one pull-box I found more than a 12 volt differential between some conduits. When the bonding was corrected, even though the bonding wires were in place, the conflict monitor tripping problem disappeared.

You might think that a standard with its reinforcing steel and anchor bolts would provide a good ground, likewise for a metal conduit. It might, but even if it did, there can be a considerable voltage drop across the ground path through the earth. That’s why the bonding is important.

Now that most newer installations utilize non-metallic conduit, bonding is still at least as important.

I don’t want to ramble on too much about this, just bring the thought to your attention for consideration.