October must be "Safety Month," with several articles related to safety in that issue of Cabling Installation & Maintenance. It is a worthy subject and especially necessary, with so many new people entering the communications cabling field.
Your October column on grounding an EMT system is especially needed. So many communications cabling workers do not see the necessity of grounding fiber-optic cable, but codes on both sides of the U.S./Canada border agree that any cable containing metal must be grounded. I tell my students that we ground systems for two reasons: to ensure personal safety and to protect the cable and components from damage.
In armored cable, the armor can build up a static charge of thousands of volts without being connected to anything because of its proximity to high-voltage cables or because of lightning or other types of static discharge.
I know of one 40-kilometer, buried fiber-optic cable system that was destroyed by lightning. Seeking an easy path to ground, the lightning actually blew holes through the armor and jacket. The installing company argued that it was grounded, and so it was. The system had a rod driven at each point of termination. However, in that particular area of the country, the ground is mostly rock, so a good connection to ground is not easy to get. Often, we have to install more than one rod or possibly a ground mat. This particular company is among the "grounding-converted," but at a high price.
Electricity, like water or air, seeks the path of least resistance. For that reason, we have to provide it a path to ground that it can`t resist. I use only #6 AWG stranded copper for grounding. It should also be attached to the central ground point, if possible. Usually, we find a loose or corroded clamp on the system, so it is wise to include a few dollars for grounding upgrading in our estimate. As tradespeople, we have a moral and legal responsibility to leave the system safe.
It is worth paying an extra buck or two for a decent ground clamp that will stay tight. In a wet or possibly corrosive atmosphere, I coat the cable end and clamp with something to seal and conduct, such as Nolux or Pentrox.
If the cable armor is in a very-high-frequency field, it might be wise to use a fine-stranded wire, since the conduction to ground is more on the surface of the copper.
Some are of the opinion that only one end of the armor needs be grounded. On instrumentation cabling, this is normally the case and is quite proper. There are installations where I might argue the merits of that idea--for example, where the cable is installed in high-voltage fields and grounding both ends would cause higher-than-acceptable currents to flow in the armor. Some utilities use aerial dielectric self-supporting (ADSS) cable because of this, even in underground applications.
Incidentally, ADSS cable can have a temporary grounding requirement for installers under conditions such as moisture on the cable jacket, voltage level of the supply, low separation distance from the phase conductors, and pollutants on the cable jacket.
In such cases, the cable`s outer jacket should be grounded 3 to 5 feet on each side of the area to be touched and connected to the nearest structural ground. In instances where the cable is being installed under active phase conditions, the cable and sheaves should be grounded with a running ground.
Unfortunately, deregulation is putting a lot of untrained people in places where I don`t think they should be.
As for electrical metallic tubing (EMT), it is only as grounded as the set screws. Often they are not all tightened. For this reason, I like to run a separate ground in EMT.
Grounding materials and labor cost money but represent only a very small part of the job cost. Without proper grounding, we risk personal safety and system destruction. That is an unacceptable cost.
William Graham, president
Mississauga Training Consultants
www3.sympatico.ca/mr.bill.graham