Do we need isolation between PLC and Field instrument

Hi, Pravin,

Sounds like you've got a reasonable question in there somewhere, but I can't quite find it. Can you please explain a bit more clearly? Posting a sketch of your system (just one input point) showing the power supply and signal routes would help.

This thread from Sparkstation (bless him wherever he is) explains how to post images.

John

hi - correct me if I am wrong

to my knowledge you need to have isolator because when you see in the case of earthing as there are two different types of supply in the same loop there will be difference in power flow in this case you need to have isolation such that this difference will not effect the earthing system....

regards

foreveryone

The output signal might be galvanically isolated from the power leads, in which case, no problem. Check the manufacturer's information supplied with the equipment.

Hi there,

You will use an isolator (galvanic) if the instrument is in a hazardous (zoned area).

Another point worth making - do not run your power supply and your instrument signal in the same cable, or in the same rack - you will only open yourself up to interference.

It sounds as if your instrument is not loop powered, in other words the instrument is supplying the 24VDC for the loop in the signal cable. If this is the case, you need to check your PLC/DCS analogue input card. Some of them allow for this type of connection (refered to as a 4-wire loop). If this is the case, you do not need an isolator.

If the input card does not allow for this, then you will need a signal isolator.

You can look at www.weidmuller.com for more information on a signal isolator.

Regards,

Craig

<...You will use an isolator (galvanic) if the instrument is in a hazardous (zoned area)....>

Eh? Not if the method employed to prevent incendive sparks from igniting the atmosphere complies with European standards.

I would recommend it and it maybe as simple as using ceramic capacitors and ferrite beads as inductors in a "T" network. It really depends on the frequency of the conversion from AC to DC and the relative load of the instrumentation with respect the control loop.

Times ten rule applies to this one!

Hi Gents,

Thank you all for your replies.My concern is, for instant let us consider a mag flow meter which has 110/220 VAC supply what will be the voltage in that 4-20mA output wire to PLC.

It will take time for me to check practically in field , Please let me know .

regards

Pravin

No, 4-20mA is Loop current not voltage and the low (Zero) signal level being 4 mA is part of the standard for noise immunity since noise current at 0 mA would be much more of a factor than noise current at 4 mA. The second very important part is that it is "Loop Current" and running both wires of the loop through a two hole ferrite bead should be all you need.

<...what will be the voltage in that 4-20mA output wire to PLC...>

Whatever it needs to be to overcome loop resistance plus the input resistance and supply 4-20mA. For a current loop, the voltage is largely irrelevant.

Hi there,

Chances are that you will have between 16 - 24VDC in the 4 - 20mA line.

Regards,

Craig

Hi craig

Thanks for your reply.Oops!! then if the plc also giving power supply(24 VDC) to that loop either one should be isolated otherwise its going to damage the instrument/module rite.

regards

Pravin

If the instrument is supplying power to the loop is is normally the case with mag meters, the the PLC must not be configured to also source 24VDC. The PLC card needs to be set to sink and not source. You also need 2 conduits to the mag. One to provide power to the mag and one for the 4-20mA loop. The only isolation I typically use on the 4-20 loop if I am not installing it in a hazardous area is a fuse on the + line before the PLC input.

Using a separate power supply, rather than the PLC power supply, is highly recommended.

Hi there,

If the PLC analogue input card is giving power to the loop and the instrument is giving power to the loop - you need a signal isolator.

You should not damage anything if you connect it up without a signal isolator - the loop will just not work. Remember it is the current that destroys the channel, you are limited to 22mA either way.

I have found (with Siemens equipment) that if I connect more than one 4 wire loop to a card configured for 2 wires that it pulls the other channels down (the voltage).

Another thing to check with 4 wire loops is the grounding. If done incorrectly you will destroy your input card as the current will pass through the channel to ground. You will have to refer to your equipment specific manuals for this.

Regards,

Craig

Hi gents

From my past experience working with Honeywell EPKS - DCS, in their FTA card they hv a option for changing the connection in this type of suitation.But in my current project we using national instrument PAC - I/O modules, anyone worked on NI-PAC ,do they hv these options. I read ther manuals they hv given some circuit connection i want to be clear whether its for the same scenario otherwise i need to provide external isolator.

regards

Pravin

<...two source power in same line...>

....is not recommended. It is always better to put the power supply in one cable and put the signal return in another. For that reason, most magflos have 2 cable entries.