Is Conduit Ground Good Enough for Electronics?
Conduit, while being a life-safety grounding system, has too much resistance to be of any use for grounding signal systems. The conduit can pick up electrical power from the building ground system, and transfer that electrical power to electronic signal systems.
For safety purposes, the one volt conduit ground voltage isn't even detected, but when electronic equipment is referencing a 5 volt signal to ground, a 1 volt conduit voltage is an interfering signal.
By Douglas Krantz
The National Electrical Code (NEC) is concerned with fire safety (after all, it's published by the National Fire Protection Association) and with electrical safety (to prevent electrocution). The electronic technician is concerned with those issues, but the electronic technician is also concerned with signal transmission at low power and low voltage.
When the equipment is using zero volts building ground as a reference for low power signal transmission, close to zero voltage as a ground reference for safety concerns is far from adequate.
For the National Electrical Code, the building ground needs to be good enough to trip a circuit breaker. If building ground has a volt or two riding on it, usually there's no harm to anything. There isn't any real shock hazard or fire hazard.
Electrical current riding on the ground is a problem though. According to Ohms Law, any electrical current on the building grounding system means that there will be different voltages at one building ground point as opposed to another. In any building, this voltage difference can be as much as 1 to 2 volts, sometimes it's higher.
On the building ground, if the voltage is different from one place to another, if the phase of the voltage is different one place to another, if the shape of the voltage envelope is different one place to another in the building, at each end of the signal carrying wire, the low power signal will be referenced by to different ground potentials.
This is a "ground loop". This is where the building's stray ground voltages interfere with signal transmission.
As an example of the problem
Two volts of ground potential difference for a light bulb or a 110 volt electric motor is not going to mean anything. On the other hand, 2 volts of ground potential difference is a major problem for electronic signals that (for data transmission) consider 0 to 1 volt as data-bit zero and 3 to 18 volts as data-bit one.
Two volts of ground difference here means that in the induced electrical noise of ground loop interference, the data will be lost.
Microphones are far worse. Their voltage is measured at hundredths of a volt. Having 1 volt of ground loop interference means that the signal from a microphone is so far down it is difficult to even measure; basically, it's totally lost in the building's stray ground voltage.
Green Wire Ground
That green insulated green ground wire, running from the ground of the electrical panel to the ground connection on the electronic equipment is important. This wire means that for signal communications between electronic equipment in different parts of the building, the ground reference voltages at each end of the wires will be much closer to each other.
Safety vs Noise Free Ground
For someone who is trained to prevent electrocution and fires, sometimes it's hard to understand the picky stubbornness of the electronic technician wanting the installed green wire. The problem is the green wire ground just seems to be redundant and conduit ground should be good enough. Installing this green wire is extra work that doesn't accomplish anything for the fire and electrical safety of electric lights and motors.
However, because the electronic technician is concerned mainly with signal transmission, and the safety aspects of electrical system are being taken care of by electricians, it's hard for the electronic technician to see that point of view.
The other aspect of all this is the tarnish buildup in conduit connectors. Tarnish takes time to accumulate.
At the time of the original installation, all the connections are going to be good; the connections will be metal to metal with all possible tarnish scraped off the connectors by the connecting screws. Conduit, however, because it is air exposed galvanized steel and not insulation covered copper, builds up tarnish in the connectors, and then it builds up non-linear resistance to low voltage signals.
This tarnish build up can take months to even years after the original installation. As the tarnish builds up, the ground loop interference caused by stray electrical currents on the ground system also builds up.
After this much time, fixing the ground loop problem becomes difficult, and often requires the installation of the insulated green wire ground wire, anyway.
Insist on the Green Wire Ground
The easiest time to install that green wire ground is at the time of original installation. If there is a problem with tarnish the conduit ground, that problem doesn't show up until months later. At a later date, installing that green wire becomes a big hassle and usually converts to an added cost to the owner. The electrician isn't going to be called back to fix the problem with the electronic equipment, the electronic technician has to find the electrical problem and then work around it.
Sometimes, at the time of the original installation, the electronic service technician or the electronic installer just has to insist on that green wire as the electrical power is being connected to the fire panel, security panel, the card access panel, etc.
Having serviced fire alarm systems for nearly 20, Douglas Krantz has compiled his knowledge of the causes of Ground Faults and how to reliably detect them into the book Make It Work - Hunting Ground Faults
. The book shows the three types of ground fault, what equipment should be used with each type of ground fault, and how to locate those hard-to-find ground faults.
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