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6.5
Safety
Most of the operator panels are fed with 24 V DC.
Power supply
230 V AC to 24 V DC
1
4
Power supply
230 V AC to 24 V DC
2
4
Power supply
230 V AC to 24 V DC
3
4
230 V AC
If a power supply that meets safety standards is used and only feeds the operator
panel, there is no problem. See 1 in drawing above.
However, if a 24 V unit that also feeds 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 feed 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 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 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 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.
Mitsubishi Electric, MA00815F
Operator panel
+24 V
0 V
Operator panel
+24 V
0 V
Distance?
Operator panel
+24 V
0 V
Small controller with expansion unit
COM1
Ch0
Ch1
COM100
Ch100
Ch101
5355
Additional Installation Tips
22
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