Two wire connection and three wire connection questions

V

Thread Starter

vuyi

My question is

1) why do we connect a pos n neg but not a ground/screen wire in a Tx but on a junction box and on DCS we do connect a ground wire?

2) what happens if the is a corrosion on terminal connections of a transmitter in relation with current and resistance?

3) a self powered loop please elaborate with an example on this one e.g where does the power comes from (source).

4) info around magnetism to understand it more in a practical manner.

Your help will be highly appreciated!!!
 
B

Bruce Durdle

> 1) why do we connect a pos n neg but not a ground/screen wire in a Tx but on a junction box and on DCS we do connect a ground wire?

First, it's a screen - not a ground. It's there to keep the signal circuit free of induced voltages from any adjacent power wires. Second, with a screen, it is important to make sure that the screen cannot carry any unwanted currents. In a typical plant, the "equipotential" voltages on the plant "earth" can in fact differ widely for a number of reasons - if the instrumentation screens are connected at both ends, the screen may carry a significant level of power frequency current which can couple into the signal leads.

> 2) what happens if the is a corrosion on terminal connections of a transmitter in relation with current and resistance?

Depends on how much and what effect it has - the basic current loop will push a constant current through a fairly high resistance without being affected. If the corrosion products end up providing a shunt path across the terminals, there will be problems - the shunt current will add to the signal current being produced by the transmitter.

> 3) a self powered loop please elaborate with an example on this one e.g where does the power comes from (source).

In a self-powered loop, then power comes from the control room end, usually off a 24 V or similar DC supply. This is connected via a fuse to the positive signal lead goi9ng to the transmitter. The return lead is then connected to the receiving equipment, usually across a 250 ohm resistor or similar. It's the loop that is self-powered, not the transmitter - the transmitter is powered by developing a voltage drop across its terminals (usually no less than 10 V is required for this.)

> 4) info around magnetism to understand it more in a practical manner.
....er.. yes
 
Top