Protect input channel AI SM 331

mikas_m · Jan 22, 2011

Hello,

how to size circuit breaker to protect analog input channel on Siemens SM 331 card? I want to place glass cb on 4-20mA current input channel. I have 2-wire transducer.
I wasn't able to find max allowable current for current inputs.

Thanks.

KalleOlsen · Jan 22, 2011

If you've got a 2wire transducer, it is powered from the AI-card itself, and you don't have to care about current or voltage limits.

Kalle

mikas_m · Jan 22, 2011

I though about it but I have experience that in the past, two analog input channels failed. I don't know the cause, but don't want to happen this again.
I'll place 63 mA circuit breaker in the loop, I hope Ai channel will not be damaged if current goes to 60 mA.

Pandiani · Jan 22, 2011

Hello,
I'm not sure if this the right way to do this.
Please, have a look at this:
http://www.allmar.send.pl/upload/files/s7300/S7300_modulesdata_en.pdf

As you can see, maximum current is only 32 mA or 40 mA (damage limit) and this 100 mA fuse is not a good choice. On the other hand I'm not aware of any quick fuse of 30 mA and never seen similar application where fuse is placed in the current loop to protect analog input channel.

Maybe you should use galvanic isolation for this.

godfrey · Jan 22, 2011

We use current isolators from Phoenix Contact.

Jobbe · Jan 23, 2011

Littelfuse sells fast acting fuses with a nominal current of 32mA.

alexyeoh · Jan 5, 2012

Wago mA ISO
model WAZ5 CCCLP 0-20mA/0-20mA shall do the work
Self power,minimus wiring work and source

HenkieD · Jan 6, 2012

We had problems where a lot of cards got blown channels during commissioning. Therefore we now setup all analogue inputs on the card side for 4 wire (passive). If needed (PT100's etc) we wire in 24V (via fuse terminal) to the one leg of the instrument, and the return goes back to the positive terminal of the AI, the negative of the channel gets hooked up to the 0V to close the loop. Since using this practice, we have saved a lot of cards and can always addon when plants are running, regardless if the new instrument is active or passive.

nonuke · Jan 6, 2012

I also use HenkieD solution. I always set the AI SM 331 to the letter "C" even if the transducer is passive.

Protect input channel AI SM 331

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