BCU 370 · Edition 01.19
68
Project planning information
6 .6 Protecting the ignition unit from
overload
The BCU protects the integrated ignition unit and the
electronic switch from overload. Excessive switching
triggers a warning signal (blinking
53
). After the mini-
mum cycle time has elapsed, the BCU starts.
The minimum cycle time saved in the BCU can be cal-
culated using the formula:
Minimum cycle time = (t
VZ
+ t
SA1
- 1) x 6
Example:
Pre-ignition time t
VZ
= 2 s,
1
st
safety time on start-up t
SA1
= 3 s
(2 s + 3 s - 1) x 6 = 24 s
In this example, the BCU 370 may not be started more
often than every 24 s.
If an external ignition unit/transformer is being used,
the formula is as follows:
Minimum cycle time = (t
VZ
+ t
SA1
- 1) x 2
Adjust the minimum burner pause time t
PB
(parameter
11) correspondingly, if required.
6 .7 Wiring
The BCU is suitable for hard wiring only. Do not reverse
phase and neutral conductor. Different phases of a
three-phase current system must not be installed at
the inputs. Do not connect voltage to the outputs.
On BCU 370..B1 for PROFIBUS DP, no voltage may be
connected to terminals 17 to 23. Otherwise, the BCU
may be damaged.
6 .7 .1 Single-electrode operation with external
ignition unit
N
13 14 15 16
PE
BCU 370
If a burner only has one electrode, which is used for ig-
nition and ionization control, an external ignition trans-
former must be used, e.g. TZI or TGI.
6 .8 BCU switched off
The BCU indicates
––
. In general, it cannot be activated
when no mains voltage is applied or the burner con-
trol unit is switched off. The fault signalling contact is
only closed when the BCU is supplied with voltage and
switched on.
BCU 370 . .B1
The bus interface is still operational to maintain the
function of the communications system. The control
outputs of the BCU (valves, ignition unit) are electrically
isolated from the mains voltage.