I have come across two LIT signals (4-20mA) sharing 1 pair through a multicore cable. Surprisingly these two level signals are being displayed in the control room.
I've never seen this before. Would somebody be able to explain how this is possible and should I be flagging this up and modifying?
The DC power for both of the loop powered transmitters comes from a common power supply.
Somehow or other, the DC power supply is wired to each of transmitters' (+) terminals, but that wiring is not using either of the conductors in the pair you've referred to. Some other pair or other wiring carries the DC supply voltage.
Each of the transmitter's (-) terminals uses one of the two conductors in the pair you referred to, and is wired to its respective analog input (+) terminal. The AI (-) is wired back to the DC power supply (-).
Thanks for your reply, I thought this might be the case but after tracing the cables I found that both (+) cores feed into 1 terminal and both (-) cores feed into another terminal. which then feed into 1 core of the paired cable mentioned.
I managed to organise a quick shutdown to connect a multimeter up at the control room end and measured no current.
Regardless of the success of your troubleshooting thus far you need to sort out you signal wiring in regard to shield grounding and DC commons. Your wiring is not considered reliable.
It may be okay today, but it can certainly come back to haunt your system when minor changes are made in the field.
It is not possible to share two 4-20mA signals on a single pair cable. Perhaps the multicore cable uses two pairs, one for each signal. It is possible for a HART digital signal to be carried on the same pair as a 4-20mA signal. The receiving device can separate the primary signal (4-20mA) and any secondary measurements carried by the HART digital data, and present that as a separate signal. Also Foundation Fieldbus can share 2-16 measurements on a single pair cable, but this is digital data, not 4-20mA analog data. Finally, I have no idea what a LIT signal is, or why you believe it to be 4-20mA.
LIT = Level Indicating Transmitter
> Finally, I have no idea what a LIT signal is, or why you believe it to be 4-20mA.
I believe you have a common DC power supply, where the (+) is standard for all devices connected, and the (-) from the DC power supply is connected internally or externally to each input channel. Then, your devices have the negative cable to close the loop with the DC power supply and the input channel.
You can see this picture here to understand > http://iqinstruments.com/iqshop/technical/4-20ma_3.gif
Another possibility is the multi drop application, where you are reading the HART e não the 4-20mA.