|
|
|
|
|
|
| Return to Savvy Consumer Information Center - Home Page |
FOR YOUR CONTRACTOR'S
EYES ONLY
(But it's OK to
take a peek)
The problem of shifting reference potentials
According to insurance company statistics, two kinds of appliances are at the top of the list as the most frequently damaged during a lightning storm: video systems (receivers and/or VCRs) and computers with a modem connection. There is at least one explanation for that very unhappy situation: if one of the two systems -power or communications -brings a surge to the house, the intended operation of surge protectors creates a difference in the potentials of the references across the two input connections of the appliance, causing failure.
In the set-up of Figure 1, a personal computer (PC) is connected to both the power system and the telephone system, and we assume that a surge is coming along the telephone wires. The surge current ("Surge I" in the figure) flows from the telephone system toward the common grounding point via the network protective device (NID) at the point of entry, and the grounding conductor mandated by the National Electrical Code (NEC ®). This current produces a magnetic field that couples into the loop formed by the power branch circuit, the telephone premises wiring lines to the PC, and the bonding conductors. A voltage (Vdiff) is induced in the loop and appears across the power input and telephone input of the PC, with upsetting or damaging consequences. The presence or absence of surge protection on the AC side (Arrester or SPD) has no effect on the coupling. One effective solution is to have the consumer install a combined protector.
 |
Figure 1 |
Even though this installation complies with the NEC, the situation is made worse if the point of entry for power and the point of entry for communications are at opposite ends of the house, with a large loop separating the two cables. The 1999 NEC now limits the separation between points of entry to 20 feet (7 meters) for new installations. The situation is even worse yet when the incoming service, cable TV in particular is not bonded to the power service ground. That is a clear violation of the NEC but experience has shown that it is not so rare, and the result can be severe damage to the appliance.
NEC© is a registered trade mark of the National Fire Protection Association
Recommended integrated bonding
The recommended bonding arrangement shown in Figure 2 is applicable for new construction or for existing homes where an opportunity occurs to relocate the point of entry of the cable TV or the telephone NID. It is compliant with the NEC minimum requirements for safety, and will reduce the problem of shifting reference potentials just described by inter-system bonding of all utilities serving a residence. The usual components for each service connection are simply installed next to each other.
Another possibility for reducing the shift in reference potentials is to install an integrated, multi-utility surge protection at the breaker panel, as shown in Figure 3. This device includes appropriate surge protection for all three services: AC power, telephone, and cable TV, with the minimum length for the bonding arrangement.
| Figure 2 Recommended Inter-system bonding |
 |
|
Figure 3 |
 |
| Return to Savvy Consumer Information Center - Home Page |
|
|
|
|
|
|
|
Get the Savvy Consumer Newsletter! (FREE)
|
|
