Additional Installation Tips
7.5 Safety
Most of the Panel PCs are fed with 24 V DC.
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
Panel PC
Panel PC
Panel PC
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 you use a power supply that meets safety standards and only feeds the Panel PC,
there is no problem. See 1 in drawing above.
However, if you have a 24 V unit that also feeds other units, there is reason to be
cautious, see 2 in drawing above. The Panel PC 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 feed is secure, for example, SELV
according to EN 60950 (protection against electric shock) and UL 950.
Example:
Here is an example that explains why a secure 24 V DC feed 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 Panel PC.
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 feed to the ground, see 4 in drawing above. This offers
three advantages:
•
Safety is increased. The 24 V feed will not be 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, MAEN043
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