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Topic Title: 24Vdc circuit protection
Topic Summary: Control panels
Created On: 28 January 2010 10:30 PM
Status: Read Only
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 28 January 2010 10:30 PM
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scrumpy

Posts: 55
Joined: 26 October 2005

Over the years, I've seen several different methods of circuit protection for 24Vdc circuits in control panels. Examples being:

Double pole MCB on PSU 24Vdc output protecting all control circuits

Each field component protected by own fuse

Groups of field components protected by single fuse

Each PLC I/O card protected by own fuse

No 24Vdc circuit protection at all

I was interested to see how other people involved in panel manufacture & maintenence approach this.
 29 January 2010 12:03 PM
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ArthurHall

Posts: 737
Joined: 25 July 2008

I would expect to see overcurrent protection at source. Single pole if one line is earthed. Double pole if no earth or centre tap earthed. Check that the device is suitable for DC, at higher voltages you often have to connect devices in series as the contact gap is too small to break a DC arc.
 19 February 2010 10:42 AM
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deleted_1_Iangreaney

Posts: 2
Joined: 18 February 2010

I have alaways used MCB protection for protection of the PSU Outputs, usually double pole on the +24VDC line. Many of my clients have their own set of drawing standards which will include all, or some of the methods listed by Scrumpy.

If on the rare ocasion I have a freehand at specifiying! then I would go for the following:

Double pole MCB on the main +24VDC line supplying suitably rated single pole MCB's for the following:

"General Control" - All Control Lamps and Sensors etc

"PLC Supply" - Power for PLC Supply only

"PLC Inputs"- All PLC Input supplies (one or more MCB dependant on size of PLC and your choice of isolation)

"PLC Outputs" All PLC Output supplies (one or more MCB dependant on size of PLC and your choice of isolation)

"HMI / Instrumentation" - as stated.

"Safety" - all Safety specific equipment PILZ realys / light guard supplies etc.

And, cost as always plays a role, some clients opt for fuses (usually the utility companis I work for!) as this is the cheaper option!

Hope this helps
Ian.
 02 September 2010 03:52 PM
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eclectica

Posts: 63
Joined: 16 August 2010

Have seen self resetting multifuse/polyfuses (e.g. Bourns) too for lower current apps (<5A). (On boards within CP that is.)

Regards
E.

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