Safety - Mitsubishi Electric E1000 Series Service And Maintenance Manual

Hardware

3.10.5 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
1. If you use a power supply that meets safety standards and only feeds the operator
panel, there is no problem.
2. However, if you have a 24 V unit that also feeds other units, there is reason to be
cautious.
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.
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 "clear-
ances 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.
3. 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.
4. Connect 0 V on the 24 V feed to the ground. This offers three advantages:
–Safety is increased. The 24 V feed will not be live in the event of a faulty connec-
tion 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.
54
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
COM100
Ch0
Ch1
Ch100
Ch101
5355
Mitsubishi Electric, MA00758A