Matrix Visionpro System Manual page 19

Input Protection Modules (IPM)
Install IPM on the Krone Modules of the MDF. Input protection modules are for analog input channels to protect
against over-voltages that may be applied between any two input connectors or between an input connector and
the ground.
Primary Protection Module - PPM4
Matrix provides Primary Protection Modules (PPM) consisting of four PPM circuits. The PPM4 contains Gas
Discharge Tubes and Fuses.
The Gas Discharge Tube is an over voltage protection device. It has three terminals. It is connected parallel to the
CO Line or the overhead station cable. The third terminal is connected to a telecom earth. When the voltage
between any of the two terminals exceeds the permissible limit (general 150V), the gas in the device begins to
conduct and the terminals with the earth terminal. Heavy voltage passes to the earth instead of entering the
system, thereby protecting the system.
The Fuses in the PPM4 are an over current protection device. Whenever the current builds up beyond the
permissible limit, (generally 100mA), the fuse opens to protect the circuit ahead.
PPM4 must be properly earthed to work well. It is recommended that PPM4 be connected to a separate telecom
earth (ground).
Telecom earth is a dedicated earth (ground) only for the PBX. A dedicated earth greatly reduces the risk of back
voltage.
13 Matrix VISIONPRO System Manual