Additional Installation Tip s
6.5 Safety
Most of the operator panels 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
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 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 dra wing 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:
Hereisanexamplethatexplainswhyasecure24VDCfeedcanberuinedbymixing
24Vrelaycontactswith23 0VACrelayconta ctsinasmallercontroller. Checkthatthe
clearancesandcreepaged istancesbetween24VDCand230VACfulfillEN60950or
UL950. Ifnot,inputaseparate24Vunitintotheoperatorpanel.
If there is a substa
ntial distance between the relay contacts for 24 V DC and
230 V AC, it is OK t
ousethesame24Vdevicesforallfeeds. See3indrawing
above.
Connect 0 V on the 24 V feed to the ground, see 4 in drawing above. This offers
three advantages:
• Safety is incr
eased. The 24 V feed is not live in the event of a faulty connection
or short circ
uitbetween0V(24V)and230Vphase.
• Tr a n s i e n t
s on the 24 V feed are connected to the ground.
• No risk tha
t the 24 V feed is at a high level in relationship to the groun d. This
is not unu
sual since there is high static electricity.
BeijerElectronics, MAEN181B
23
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