Transcript
Network Video Technologies
Technical Bulletin Protecting Against Surge/Transients At high voltage, surge is identified as a lightning hit or other transient. That is large currents traveling through the wire over to the other end’s ground, damaging equipment along the way. For protection, leaving the camera floating is a far superior approach when dealing with voltage surge than is local grounding. This “floating” method will require rubber pads or plastic screws, but it’s well worth it in the lightning prone areas. Parking-lot cameras on metal poles or those mounted on metal-skinned buildings are particularly vulnerable. If a transient protection device is used, place it right at the camera. Connect the protector’s ground wire to the camera’s chassis (or shield of the BNC connector) rather than the local ground, to prevent transients back-flushing into the system. This “single-point grounding” practice is highly effective for both high voltage transients and low-voltage ground loops. These practices should always be subject to regulatory safety requirements such as Electrical Code.
Transient Protection Theory Remember the Vandegraff static electricity generator in your high school science class? (That was the machine that made students' hair stand on end.) The smooth dome allows a high voltage while minimizing the field density. If you were to shut off the machine and attach a pin to the top, and then turn the machine back on, the end of the pin would arc like crazy. This is the principle behind lightning rods. Now think of the top of a pole out in a parking lot and imagine that the earth is the smooth dome. The pole is sharp and pointy. Just like the pin, the electric field density is very high at the top. That makes it a great target for lightning strikes.
Low Voltage Camera* BNC Video Out
NV-214A-M Monitor
NVT Receiver Power (when required)
DVR
* Unless prohibited by Electrical Code, the camera and NVT should float, relative to earth ground. This will reduce sensitivity to lightning strikes. †Be sure that ground rod is not shared with other equipment that may dump large currents, such as telephone company entry protection devices, lightning rods, metal-skinned buildings, or radio towers.
24 VAC
Primary ground point shared Supplemental Ground with receive-end. rod (optional)†
For the sake of discussion, let's say the voltage at the top of the pole is 50,000 volts. The bottom of the pole is at 50,000 volts too. If the pole is grounded, the top of the ground rod will be at 50,000 volts. And the bottom of the ground rod will also be at 50,000 volts. (Dirt doesn't conduct well.) If you were to sink a new ground rod 5 or 10 feet away, this rod might be at 49,000 volts. Now, if a camera is powered from a low voltage source, there is no life-safety, electrical code, or UL requirement that it be grounded. (Check the camera's manual, too.) This brings us to an important choice. We can connect this camera to a source of 50,000 volts, or not. If we do ground the camera, then current will flow through our copper back to the head-end, which is at zero volts because it is grounded. If we float the camera, no current will flow. This principle is one alarm installers know well: ground the head end; float the remote keypads - nothing gets cooked. The NVT device should be at the camera with its ground connection tied to the camera's chassis, but both floating with respect to the local ground. Ideally at the receive end, all CCTV equipment, including the NVT, should be co-located and share the same building electrical ground. Having the NVT grounded to the phone-company's ground rod, while the rest is connected to the building ground could allow large voltage differences to damage equipment. Lightning protection is not an exact science. However, customers who have experienced problem installations have successfully employed these practices with excellent results.
