4-20 mA For 500 Meter Distance

What have you modified? Has the distance changed? Is 500m the new distance, or the old? Was it previously working?

Need more information. You are not helping yourself by posting incomplete data.

500m is as nothing to a properly-installed 4-20mA current loop. If loop resistance is perceived to be a problem at that distance, the usual remedy is to increase the power supply voltage.

Dear JohnDG,

We are working in paper mill. Firstly it was 2 layer machine but now it is going to be three layer machine. For third layer we have modification project with 1150 more I/Os.As discussed with you, some of the I/Os have distance more than 500 metre from DCS. We have to decide whether we can use the same scenario or remote I/Os can be used.

Shouldn't be a problem, provided (as has been said) the power supply voltage is high enough to drive 20mA through the loop.

Makes me wonder, tho', if this isn't a bit of a retrograde step. Why have a Distributed Control System, and still run hundreds of cables back to a central location?

How many I/O's are about 500 m away from the central unit out of the 1150 I/O's?

If there will be about >50 I/O 500 meters away, in my personal opinion, having it organized by a remote I/O is preferably best. You don't need your system to get complicated with wiring and conduits just like varicose veins , further, it will be inconvenient for maintenance technician to trouble shoot a system (when malfunction) having a distance considerable enough to be tiring from walking back & forth.

Having a remote I/O will let you do the troubleshooting job isolated and convenient.

Depends on the cable specification. Check the voltage drop calculation for 500m with cable specification, calculated voltage shall not be bellow than instrument required minimum voltage.

As indicated by others if you up the voltage feed and make the cable big enough you might get good signals back.

If you want you could use an I/O with a fiber optic link and then it will work also.

The issue here is not really the distance rather the cost of so many cables coming and going, which tend to be expensive.

We have connected transmitter at around 1.2 km from DCS with 24VDC power supply and there was no issues at 20mA also. A transmitter requires around 12-15 volts for proper working and there wont be any potential drop across barrier. So 500mtrs won't be a problem and try to connect a sample transmitter at that distance to decide yourself

The real challenge is not whether the 4-20 can transmit over that distance also care can be taken to see that the voltage drop does not cause a problem through proper wire size selection, but the installation cost of wiring many devices even in home run cabling efforts can be lowered by using remote IO... Many systems now offer Ethernet versions that allow faster transmission paths and if they support fiber then noise issues are also reduced to only the areas where the transitions take place. The justification has to consider the area classification for the remote IO and if it can be placed centrally in the IO to minimize wiring runs this can save installation time and startup time if engineered properly... The breakpoint on remote IO can be as low as 60 IO depending on area classification and utilizing the best method of protection for the installation such as intrinsic safety methods over explosionproof for division 1 areas...

If you have only three loops with distances of 500mtr from your control room, then you don't need to put any remote I/O. If the I/O becomes more than 50, you need to install a remote I/O panel to avoid the cost and cabling only.

My Experienced, I've installed for annunciator distance 720m with regular control cable 2,5 mm shielded one tray with other DC cables. It has bad measurement, some like intermittent, first we suspect the gauge is damaged. We changed but still problem. So we change special for instrument cable, its very expensive !...and done, all signal never get traffic jam..means since that every thing its OK