Additional Installation Tips
7.5 Safety
4
1
2
4
3
4
Power supply
230 V AC to 24 V DC
Power supply
230 V AC to 24 V DC
Power supply
230 V AC to 24 V DC
Operator panel
Operator panel
Operator panel
Distance?
Small controller with expansion unit
+24 V
+24 V
+24 V
0 V
0 V
0 V
230 V AC
COM1
COM100
Ch0
Ch1
Ch100
Ch101
5355
If a power supply that meets safety standards is used and only powers the operator
panel, there is no problem. See 1 in drawing above.
However, if a 24 V unit that also powers other units is used, there is reason to be
cautious, see 2 in drawing above. The operator panel does not have insulation
that meets safety requirements in the event of a potential short circuit between
230 V AC and 24 V DC. It is assumed that the 24 V power supply is secure, for
example, SELV according to EN 60950 (protection against electric shock) and
UL 950.
Note:
Here is an example that explains why a secure 24 V DC power supply can be ruined by
mixing 24 V relay contacts with 230 V AC relay contacts in a smaller controller. Check
that the clearances and creepage distances between 24 V DC and 230 V AC fulfill
EN 60950 or UL 950. If not, input a separate 24 V unit into the operator panel.
If there is a substantial distance between the relay contacts for 24 V DC and
230 V AC, it is OK to use the same 24 V devices for all feeds. See 3 in drawing
above.
Connect 0 V on the 24 V power supply to the ground, see 4 in drawing above. This
offers three advantages:
•
Safety is increased. The 24 V power supply is not live in the event of a faulty
connection or short circuit between 0 V (24 V) and 230 V phase.
•
Transients on the 24 V feed are connected to the ground.
•
No risk that the 24 V feed is at a high level in relationship to the ground. This
is not unusual since there is high static electricity.
Beijer Electronics, MAEN332
25
