1.7.2 Power supply printed circuit board
The power supply pcb is retained inside the alarm panel back box with
four retaining studs and connects to the light display pcb multi-way
ribbon cable by polarised connectors. The power supply pcb
incorporates four mains terminals (EENL) connected to matching
plug/socket combination to accept the mains electrical power supply,
which preferably should be from an essential circuit, and enables
connection of flying earth leads which electrically bond the assembly.
The power supply pcb is programmed during build to accommodate
the electrical power supply voltage which must be specified at order
stage. A filter protects the alarm system from possible spikes or
disturbances of the incoming electrical power supply and an integral
transformer provides 15V and 5V d.c. supplies to operate the alarm
circuits. Three fuses 20mm x 5mm to BS4265 protect the power supply
circuits. F1 is rated at 500mA for 240V supplies and 1A for 120V
supplies, F2 is rated 500mA and F3 is rated at 500mA. All mains power
supply circuits and components are covered by a clear plastic shield,
retained by two studs. A warning notice reminds personnel to isolate
the electrical power supply prior to removal and maintenance. In
order to prevent inadvertent cross connection, the mains electrical
power supply plug and socket is not compatible with any other
connection to the pcb.
The power supply printed circuit board incorporates a microprocessor
which controls the operation of the alarm panel. A miniature
‘maintenance’ push button is fitted on the power supply pcb and is
accessible with the alarm panel front cover open. With an alarm
condition displayed, operation of this button ‘permanently’ disables
audible reinstatement for that particular displayed alarm condition
only. This facility is designed for use when the plant or pipeline is shut-
off for a prolonged period. On returning the pipeline pressure to
NORMAL this feature automatically re-sets without further manual
selection.
The power supply pcb incorporates 5 input channels for connection to
their respective inputs. Each channel (designated CH1 to CH5)
incorporates 5 terminals (C – Common, 1, 2, 3 and 4) which are
connected by matching plug/socket combination to accept signals
from their respective inputs.
A relay (complete with line contact monitoring circuit) suitable for
switching 50V and a maximum of 0.5 Amps is fitted to the power
supply pcb. The normally energised relay has volt free, normally closed
contacts and two terminals (N/O and C) enable connection by a
matching plug/socket combination to either a Medipoint central
alarm system or other suitable system. The line contact monitoring
circuit can be de-selected by means of a jumper on LK4 located on
the power supply pcb. The relay is de-energised and contacts open
when any of the twenty alarm conditions are initiated. Terminals are
also provided to enable connection of a remote audible warning
device (+ and AL).
It is recommended that the input and data transmission cables are
installed separate from the mains cable and maximum cable length
between alarm panel and input should not exceed 100 Metres. Alarm
panels accept a mains electrical power supply cable of 2.5 sq mm,
and all other terminals accept a maximum cable size of 1.5 sq mm.
1.7.3 Standby battery
The power supply pcb also contains a standby battery, battery
charging and power fail detection circuits. The battery provides power
for both the SYSTEM ALARM indication and the audible warning in the
event of an electrical power failure. The battery is fully charged after
72 hours and provides sufficient power to operate the specific alarm
indications for a minimum of 4 hours. The battery is expected to have
a minimum 5 years life.
1.8 Alarm contact line fault
CAUTION
Only Hill-Rom MEDÆS line contact monitor modules should be fitted,
otherwise the SYSTEM ALARM circuits could provide spurious indications.
Integrity of the interconnecting wiring and input sensors are constantly
monitored by the SYSTEM ALARM detection circuits. The monitoring circuit is
designed to detect an open or short circuit as well as normal operation of
the pressure sensor. In the event of a line fault the NORMAL LED remains
illuminated, the appropriate gas service alarm indicator and SYSTEM ALARM
indicator will illuminate and the audible warning sounds. When a line contact
fault is detected the flash rate of the affected circuit LED speeds up to show
short circuit or slows to show open circuit when the TEST switch is operated.
This facility is designed to aid fault diagnosis. Inputs must be ‘normally closed’
volt free contacts.
1. DESCRIPTION AND
OPERATION
1.7 Printed circuit boards
continued
