10
EN2R--9023 9705R0--NE
SAFETY PROVISIONS
15
8
16
A4021A
12
13
= input micro--processor
LGPS
H.D.
valve--1
valve--2
14
burner controller
V2
V1
H.D.
input
out
vent--valve
external reset
2
L
11
alarm
6
N
int. power supply
H.D.
post--config.
post--config
pre--config.
1
GPS
9
3
4
10
5
7
pre--config.
fuse
16A
N
= external wiring
= optional
internal fuse
5 A slow
Fig. 9.
Simplified internal schematic and wiring diagram pre-- and post--test configuration.
Energizing valve 1 and 2
The hardware of the A4021A prevents that during the valve
proving both valves can be opened at the same time.
Use of terminal 12
With terminal 12 of the valve proving system the heat demand
status is checked by the valve proving system in post
configuration. For the pre--configuration it is necessary that
terminal 12 and14 are connected.
If terminal 12 is not connected, the A4021A will raise an alarm
(error heat demand).
Fail--safe relay drivers
The fail--safe relay drivers which are used to give the
heat--demand to the burner controller are fail safe.
Anti--recycle--counter
For reason that during each valve proving a small amount of
gas enters the combustion chamber, extra safety provisions
are build in to prevent the A4021A from continuously
recycling, in case of failures of the complete burner control
system. This safety feature is provided by an anti--recycling
counter in the A4021A. When the heat--demand signal
disappears during a valve proving or within 10 seconds, after
the valve--proving is finished. This leads to lockout (error
self--test) of the A4021A when this behavior occurs 10 times
in a row. The counter will be reset by the first heat--demand
which lasts longer than the A4021A total--test time + 10
seconds. In this way the A4021A offers additional safety.