EXTER EXTER T150sr, Installation Manual

Additional Installation Tips

Beijer Electronics, MAEN966

23

6.5 Safety

Most of the operator panels are fed with 24 V DC.

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.

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.

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 operator panel.

4

1

2

4

3

4

5355

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

+24 V

+24 V

+24 V

0 V

0 V

0 V

230 V AC

COM1

COM100

Small controller with expansion unit

Ch0

Ch1

Ch100

Ch101

Distance?