3-2
Removing the AC power cable connection from the Power Supply still leaves
primary AC power on the terminal block to the right of the supply. During
supply removal it is possible to contact the primary AC power which is observed
on many power supplies and is evidenced by arcing marks on the top surface of
the heat sink.
Upgrades should not be performed with system power on. This practice
damages Mapnet and NAC circuits by backfeeding AC power into circuits
powered from the A & B taps. This over-voltage condition accounts for
significant Power I/O Interface and Standard Slave Mapnet & I/O circuit
damage.
Over-voltage in excess of 35 to 40 volts destroys the Power I/O Interface MOV
transient suppressers between the A & B taps and 24C ground and/or between
the 24C ground and earth.
Contractors should understand how to properly wire peripheral devices handling
AC power.
The fuse wiring harness (Part No. 733-731) referred to in the 4020 Field Wiring
Diagram (841-842, sheet 22) is available to limit B-tap over-voltage damage..
Most Master Controller PCB assembly failures appear to be the result of
electrostatic discharge (ESD) into the Master Controller. ESD may be entering
the system during the installation of the U8 flash device or from contacting the
installed flash chip during connection of the download cable onto the adjacent
P3 header connector.
Note:
Static electricity can damage components. Use the following
precautions:
•
Ground yourself before opening or installing components
(use the 553-484 Static Control Kit).
•
Keep uninstalled components wrapped in anti-static material
at all times.
System ground faults are normally due to field wiring problems, but it is
possible for ground faults to occur within the power supply. Excess leakage
current on capacitors C36 and C38 of the Power Supply Universal Switcher
PCB assembly (Part No.565-235) has been known to cause system ground
faults.
On at least one occasion it was reported that the drop panel retention wire
insulation was fractured and the wire shorted to one of the PCB assemblies. In
this instance, the ground fault disappeared whenever the front panel was opened.
Continued on next page
General Issues
Damage Caused by Live AC
Power
Damage Caused by
Upgrading Live Circuits
Damage Caused by
Electrostatic Discharge (ESD)
Ground Faults
