It has been nearly two decades since incompatibility between the National Electrical Code requirements and the design criteria for a high reliability of electrical supply resulted in the addition of Article 708, Critical Operations Power Systems.
However, this article was not intended as a how-to for designing an electrical system to minimize the effects of power quality phenomena. NEC Article 90.1 clearly states that the Code’s purpose “is the practical safeguarding of persons and property from hazards arising from the use of electricity” and it “contains provisions that are considered necessary for safety.”
It also states, “Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.”
Even after Article 708 was adopted, there are still those who say that all that they need to do is follow the NEC requirements and the equipment should operate properly, which isn’t always true. The following example shows on a small scale what could potentially be a safety hazard and equipment misoperation or failure due to not following the Code. It does illustrate many of the problems found in the field that affect the quality of the electrical supply, such as illegal neutral-to-ground bonds.
A picture speaks a half-dozen violations
The picture is from a two-story colonial house, originally built in 1942, with a major renovation 15 years ago. The house recently underwent another renovation to remove the wall between the kitchen and dining room and support with two microlam beams. Three switches and two receptacles in the wall needed to be relocated. One of the switches was a three-way, which was part of a circuit in the kitchen.
On a wall in the kitchen near the side door were two metallic single-gang boxes screwed together to make a double-gang. The left side housed a dimmer switch (not shown, but it was connected to the red, black and blue-taped wires heading off the picture), and a three-way switch that had one of the black wires broken off the screw terminal. The dimmer controlled a ceiling chandelier and the three-way switch was part of the circuit that controlled two recessed lights over the counter. Above the “double-gang” box was a display for the alarm system, which needed power.
To the right is a single-gang, nonmetallic, low-voltage old work box with two wires to a receptacle. Since there were no receptacles nearby on that wall, someone added this box and ran white and black wires over to the metallic box (without a strain relief). To hide the power supply wires for the display, they ran them through the wall and over to the side of the low-voltage box and out the side.
When the cover was on the box, the wires were protruding slightly out the bottom, and then the power supply was plugged into that receptacle. Perhaps because the power supply didn’t have a ground pin, they decided it wasn’t necessary to run the ground wire over to the receptacle. Instead, they twisted and capped the ground wire pigtail from the grounding screw in the metallic box to the neutral wire.
Power supply circuit wiring
Though there are at least a half-dozen NEC violations in this example, the display power supply circuit wiring is the most concerning from a PQ perspective. While this power supply had no grounding pin, there is no guarantee that something else plugged into it in the future would not have one. The grounding conductor allows for a path for surge protection devices to divert the transient energy to. Though this display device communicates wirelessly to the rest of the system, some security alarm systems are hard-wired to the phone system. When a lightning strike couples the energy into the phone wiring, having no ground or surge protection can lead to destructive results throughout the house.
The neutral (or grounded) conductor is only to be grounded at the service entrance bonding point. When there are secondary bonds between them, the current in the neutral conductor now has multiple paths back to the service entrance bonding point. Normally, the current in the grounding conductor is very small, but put the half of the load current of the neutral into it, and the equipment ground will have an elevated voltage. In addition, fault current carried in the grounding conductor can now make its way into the neutral conductor and equipment powered by it.
This situation was one of a dozen in this house demonstrating why only licensed electricians should be doing this work. It took the electricians an extra week to correct all of the problems while trying to relocate those few switches and receptacles.
Image credit: richard p. bingham
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.