Summary
Modern intercom systems, with solid state circuitry, are much more efficient and compact than older systems with relays, coils, and vacuum tubes. Unfortunately, they are also more susceptible to malfunction or failure due to transient overvoltage, such as lightning and AC power surges.
Lightning
Lightning overvoltage is caused by a strike of lightning to or near a conductor or shield of an intercom cable. Shielded wire does not provide complete protection against lightning damage. When lightning or other voltage spikes flow on the shield of a cable, voltage is induced between the internal conductors and the shield. The amount of induced current depends on the resistance of the shield material and construction of the cable.
AC Power Surge
A typical example of power surge is the voltage spike that occurs when heavy duty generators are powered up, or when surges occur on the main electrical system. Lightning can also create power surges in AC power lines. Ranging in amplitude from harmless amounts just above normal voltage to several kilovolts. If the intercom cables are run with the AC power lines (which is strongly discouraged), the AC power surge can bleed over into the intercom cables.
About Metal Oxide Varistors
Voltage spikes affect all semiconductors, which are intolerant of voltage transients in excess of their ratings. Even a slight amount of excess voltage can cause a semiconductor to fail or be degraded enough to shorten its useful life. Most lightning damage to intercom systems is caused by lightning opening or shorting semiconductors. A voltage clamping device such as a metal oxide varistor (MOV) is recommended to absorb the voltage spike before it enters the intercom unit. An MOV has the best volt-ampere characteristics, energy absorption, and peak current capabilities compared to other voltage clamping devices. The specifications for an MOV used on an Aiphone system are as follows:
- Continuous DC voltage: 50 ~ 100 V
- Transient Peak Current: » 250 A
- Maximum Clamping Voltage: 100 ~ 150 V
One MOV is to be installed between each wire in the cable and an earth ground for each intercom unit. In lightning-prone areas and where power surges are possible, surge protection should be installed.
Modification
Prevention of AC power surge and/or lightning damage.
Difficulty Level
2 – Easy/Moderate – component connection to external points.
Components Required
- SA-1: surge arrestors, one for every two terminals being protected.
- Surge sensitive power strip.
Instructions
- Plug all system power supplies into surge sensitive power strip, then plug the strip into an AC outlet.
- Wire SA-1s on all intercom terminals as shown on the diagram, with output side connected to an earth ground.
- SA-1s are recommended to be installed on master stations only and are not necessary on remote stations.
