Preliminary Information
Protection Circuitry
Primary protection, from lightning strikes and AC line
faults, is normally located in the MDF (main
distribution feeder) which is located external to the
PABX or CO switching system. The primary
protection circuitry is normally housed in a 5-pin
connector and consists of either carbon blocks, with
spark gaps (older technology), gas discharge tubes
or high current semiconductor suppressors and
series heat coils. The 5-pin module usually limits the
high voltage to approximately 300 to 500 volts before
entering the switching system.
Secondary protection, in the switching system, is
required to further limit these high voltages/currents.
Secondary protection is normally implemented on
each line card and is designed to protect the SLICs
from permanent damage. The basic secondary high
voltage protection circuitry for the MH88620IN, as
illustrated in Figure 7, consists of PTC1, PTC2 and
clamping diodes D1 to D4. During a fault condition,
the diodes clamp the overvolt Ground and -V
Bat
.
PTC1 and PTC2 current limit as their resistance
increases with power dissipation caused by the over-
a voltage/over-current condition. The ground that D1
and D3 are connected to, must be an EDG (energy
dumping ground) which is connected to the chassis
MH88620IN
or system ground. This is a seperate conductor from
LPGND or AGND on the line care PCB. D2 and D4
conduct the energy into a -V
Bat
supply which is a
seperate conductor from the -V
Bat
feed supply to the
SLICs. A power MOSFET circuit as shown in Figure
8, can be used to divert the energy normally dumped
into -V
Bat
, the EDG conductor. Usually one MOSFET
circuit can be used for 16 SLICs or per line card.
Depending on the additional level of protection
required, PRO1 and/or PRO2 protectors may be
used. These are used to protect the SLICs Ring
sense resistor and/or Ring generator from being
damaged if a fault condition occurs during the
application of Ringing to the line. PRO2 can be
implemented using two back to back zener diodes, or
an equivalent transient suppressor. The clamping
voltage should be >16 Vdc and <26Vdc. PRO2 may
not be required depending on the value and power
dissipation of PRO1.
Loop Length
The MH88620IN can accommodate loop lengths of
up to 2000 ohms minimum (including the subscriber
equipment). This corresponds to approximately 8km
using #26 AWG twisted pair or 15km using #24 AWG
twisted pair.
MH88620IN
24
Z2
R
Internal
5500
Ω
23
Z1
MH88620IN
24
Z2
R
Internal
5500
Ω
Z1
23
External
0.22µF
MH88620IN
24
Z2
R
Internal
5500
Ω
Ζ1
R
External
340Ω
A.
B.
C.
Notes
a) to accommodate the use of 2 x 25Ω PTCs, connect Z1 and Z2 together, Z
in
= 600Ω.
b) to accommodate the use of 2 x 8Ω PTCs, connect 340Ω between Z1 and Z2 = 600Ω.
c) to accommodate the use of 2 x 25Ω PTCs, connect 0.22µF between Z1 and Z2 = 6.0Ω + 2.2µF.
Figure 3 - Input Impedance (Z
in
) setting
2-167
MITEL [ MITEL NETWORKS CORPORATION ]
