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Dear Experts,

I have a flow switch (FSL) used for safety of a pump. the FSL has one SPDT relay.

* The FSL is used as a hardwired interlock to the motor starter of the pump.

* We need to monitor the status of the FSL on the DCS as well.

* The starter is intelligent with PROFIBUS communication.

* I believe it is better to send the FSL status separately to the DCS hardwired and not only via PROFIBUS (Q1: Is it logical or it is not really necessary?)

* I know I can use a DPDT relay in the panel and send two isolated signals to DCS and the starter

* Q2: Is it alright if I connect the FSL directly in parallel to the DCS DI card as well as the motor starter DI? (I myself do not think so!)

I would appreciate if i get your opinion on Q1 and Q2 both.

Many thanks,
Hamid

Question: Do you REALLY want to know the state of the FSL or just whether the drive is running or not, may be worth leaving the FSL as is and just taking an auxiliary contact from the motor starter as a DI into DCS. Or indeed just use the profibus link already mentioned if you are comfortable doing this.

For Q2 I definitely agree that paralleling the switch up is not the way to go.

The DPDT relay external to the FSL is also a valid option, but as ever this involves yet another piece of hardware (point of potential failure) in the loop.

Is there any FT in the loop, which is being used for flow indications? If so, you can use a trip amplifier along with it to indicate the low flow condition. You will have to ensure that the trip amplifier is set at the same value in terms of 4-20 mA where the FSL actuates.

Parallel connection is a definite NO NO...

If it is imperative to have indication on DCS, you can consider non-intrusive type flow switch for your application.

You can find further details at the following thread:

http://www.control.com/1026200615/index_html

>Q1: Is it logical or it is not really
>necessary? <

A1: In my opinion, it's logical, it's necessary, and it's a good thing. It's very useful to have a "physical signal" (no Profibus, no "comm").

>* Q2: Is it alright if I connect the
>FSL directly in parallel to the DCS DI
>card as well as the motor starter DI?
>(I myself do not think so!) <

A2: In my opinion, the best choice is connecting the flow switch to the digital input of the DCS, and manage the starter DI from a DO of the DCS. With this arrangement, a lot of interlock conditions can be included in the "operating conditions", and it allows you to know the status of the "interlock chain".

>* The starter is intelligent with
PROFIBUS communication. <

- This is a better option if you have provision in the DCS to read from Profibus.

>* I believe it is better to send the FSL status separately to the DCS hardwired and not only via PROFIBUS (Q1: Is it logical or it is not really
necessary?) <

- If the requirement is only for indication first option is better.

>* I know I can use a DPDT relay in the
>panel and send two isolated signals to
>DCS and the starter <

- This will add one more component in the circuit and may have impact on the chance of failure.

>* Q2: Is it alright if I connect the
>FSL directly in parallel to the DCS DI
>card as well as the motor starter DI?
>(I myself do not think so!) <

- Not at all a good option.