Security Watch: You’re Grounded
Feb 1, 2006 12:00 PM,
By Steve Filippini
Finding a good ground can protect your equipment from surges.
My first lesson in respecting Mother Nature occurred in Miami in the late 1980s. I was standing in front of a burglar alarm control panel and was about to use a screwdriver to remove a circuit board I suspected to be defective when the house was hit by lightning. In that fraction of a second, my panel erupted in sparks and a puff of smoke flew up in my face. The components that supplied telecommunications for my system were immediately charred and unrecognizable. The aroma that wafted from the metal enclosure can only be appreciated by other field technicians who have also experienced roasted resistors and capacitors, and over-cooked transistors.
After a quick change of clothing, I instructed the local technician I was working with to remove the bad panel from the now blackened can. He called me a coward. I didn’t care. I was more intrigued as to why the internal and external surge protection supplied with the control panel didn’t work. I knew nothing could completely stop a direct lightning strike, but the damage to my panel was far more extensive than I was used to seeing. After a few minutes of investigation, I discovered my system was not connected to either an electrical or cold water pipe ground. In fact, the green, 14-gauge, solid-copper wire attached to the back of the metal enclosure only went a few inches into the wall, where the end hung loose in the empty space behind it. Clearly, this was why the internal circuitry that would have handled up to 25,000V (+/-) of an electro-static discharge (ESD) didn’t work. At that point, it didn’t matter why the panel wasn’t properly grounded. The damage was done. It was time to correct it.
Finding a good ground can be a challenge. Inside a home, it’s best to locate a solid cold-water pipe without PVC lines between the panel and the water service. If that’s not possible, then the ground plate where the electrical service enters the structure is best. The last thing you want to use is the shield that runs in the step-down power transformer wire at the panel. Too many technicians will connect the shielded wire at the electrical outlet and at the metal control panel enclosure. There are several reasons why this is bad. Connecting both ends of a shielded cable will create a ground loop and cause the cable to act as a dipole antenna, which introduces interference instead of removing it. Using the outlet ground will allow AC trash (static, noise, inductance) to flow into the panel and possibly interfere with the microprocessors controlling the system. And finally, the 22-gauge wire is too small to handle current surge protection and ESDs.
Inside a business, you have a few more options in finding a good ground source. The steel structure is an excellent path to earth ground. Electrical panels are another, but it’s always best to find the “Holy Point” — the mother of all grounds — the primary ground spot, usually found at service entrance meters. If none of this is available to you, then you will probably be reduced to driving an old-fashioned ground rod into the soil. Ground rods are fun to drive, provided you know how to handle a sledgehammer. If the ground is too hard or bedrock lies only a few feet below the topsoil, it is permissible to bury the ground rod lengthwise. This method is called a ground ring and requires a rod at least 20ft. long buried at least 30in. deep.
Regardless of what you choose for a ground source, keep in mind the resistance between the device you are protecting and the ground must be less than 25Ω. In fact, less than 5Ω is preferred. If not, the potential difference generated by a current surge will zap the protected item and all of your efforts will be lost. If you have multiple grounds for a large system, and the resistance between each ground node is varied, you will create an electrically unstable environment. Single-grounded systems are best.
A ground clamp is one of the most important parts I ship with each system, and it’s the one most frequently overlooked. I believe installers either toss them into a spare parts bucket or throw them at aluminum cans for target practice — either way, they are not ending up at the job site. I have found the most effective ground clamps are made of copper and have ridged teeth inside the curve of the clamp for maximum grip and connectivity. Ground straps are my least favorite, and for good reasons. First and foremost is the possibility of oxidation forming between the copper pipe and the strap. This builds up resistance between the pipe and panel and greatly reduces the effectiveness of the protection. My other reasons range from how the ground wire attaches to the strap with a nut-and-bolt configuration to how often I slice my hands open when handling them. A good ground clamp has a slot where the wire can rest with a large screw-down bolt holding it in place.
Why the concern for having a solid earth-ground connection? Power surges are occurring all the time in home and businesses. Every time you are sitting in front of your television or computer monitor and it flickers, you have just experienced a power surge. They only last a fraction of a second, but they can carry a 10,000V tap on the shoulder. These momentary surges may not outright destroy a powered device, but they can slowly break down the integrity of the components (integrated circuits and transistors for example) and dramatically reduce their life span. Power companies are sometimes to blame for these anomalies (loose neutral wires, grid-shifting, and power switchovers); stormy weather (lightning) is also a large contributor.
What people tend to forget about are the mini-electrocutions we inflict on our products daily. I live in Minnesota where the temperatures are known to plummet during the first six months of winter. When the air gets cold, the moisture content drops. The drier the air, the more we generate static electricity. The power created from our sweatshirts, sweaters, and wool socks across the carpet (25,000V to 35,000V!) can make even the toughest person yelp in pain and surprise when it finds a good ground source to dissipate through. Each time I touch a television control button, stereo switch, personal computer terminal, or alarm system user keypad and produce a spark that resembles a flashbulb, I am slowly destroying that item. One thing to keep in mind, though, when discussing voltages ranging from 10,000 to 35,000, the current passed on is in the micro-amp range. There isn’t enough current in the discharge to hurt you, but it will wake you up.
But, despite all of your good intentions to properly ground your systems to avoid damage, sometimes it’s necessary to enhance the level of protection by bringing in the big guns. I’m talking about surge protectors. Externally added surge protection is an added line of defense against natural and man-made disruptions. There are many manufacturers who provide surge protection, and their methods often vary. Some companies use gas-tube arrestors, while others use silicon avalanche diodes (SAD) and metal oxide varistors (MOV). These components hide at the front of circuits and wiring harnesses, waiting. When a level of current or voltage rises above a pre-determined setting, the transient detection component will open a low-impedance path to ground and safely re-route the damaging spike. Regardless of which manufacturer you use or are comfortable with, there are a few guidelines you should follow when installing these devices:
- Take your time determining the best “Holy Point” to use. Don’t settle for the one easiest to get to.
- When running a ground wire (12- to 14-gauge wire), avoid 90-degree bends in the cable. A massive current surge will be unable to “make the turn” and stop in its path to a ground point. And while we are discussing ground wires, some manufacturers require stranded wire (more surface area to handle the load) while others recommend solid (a more robust pipeline to ground). Most require the wire to be copper due to its conductive properties. My recommendation is to at least have one.
- The distance between the surge protector and ground must be shorter than the distance between the protected device and the surge protector. That way the current surge reaches the ground before it reaches the device.
- The wire between the surge protector and the device must be at least 3ft. to 6ft. in length. That way, any electro-magnetic field generated by the transient surge and protector is kept at a safe distance from the protected device.
- Do not add ground wires from individual modules of a security system to the metal enclosure. This just creates ground loops.
Preventative maintenance is always recommended for your systems. So, while you are tightening a few wire terminals and securing a few loosely mounted devices, spare a minute and take stock of your earth-ground protection. You’ll be surprised at how much money you’ll save in the long run.
Steve Filippiniis a senior technician with more than 27 years of experience in the security and installation industry. He can be reached at[email protected].