Revisiting the difference between passive dry contacts and active wet contacts? How are four-wire and two-wire systems configured and connected?

Active and passive: Active, Passive: This term applies to AI. If a field instrument requires a DCS card to supply it with 24V power, then such an instrument is considered a passive signal for the DCS. Conversely, if the field instrument already has 24V power supplied to it from another source and does not need power from a DCS card, then it is considered an active signal. Active and passive signals require different wiring; for example, in HONEYWELL’s TPS, AI cards 1 and 2 are connected to passive signals, while cards 2 and 3 are connected to active signals, meaning no external power supply is needed. In the CS3000, jumper settings are required. Dry contacts and wet contacts: These terms are used in reference to digital switches. Although “dry contact” seems to be a colloquial term, it is actually a standard term used in the field of industrial control. The dry contacts of a relay mean that the power supply for the contact circuit comes from an external circuit; the relay merely provides a \"node (contact)\\" to connect the circuit, without interfering in anything else” ； Wet contacts, on the other hand, correspond to dry contacts: in addition to providing contacts for closing the circuit, they also supply the current required in that circuit

So what about two-wire and four-wire systems? ? ? ? ?

The passive configuration has two signals and two power supplies, right?

Some of the instruments use 4-wire connections for both power and signal transmission; in such cases, 220V power is required. Another type uses a 4–20mA signaling system, with a 2-wire configuration that operates at 4–20mA levels, and it may also have 24V power supply. In terms of electricity, electric shocks can occur even without a power source.

You need to first distinguish between digital measurement instruments and analog measurement instruments

This post was last edited by tbh007 on 2015-9-25 00:05. Since contacts are mentioned, it has nothing to do with analog signals; generally, passive contacts refer to DI signals. Dry contacts come in two types: mechanical and electromagnetic. Mechanical ones, such as the auxiliary contacts on contactors, operate through mechanical action, so when wiring them only one power line is needed, along with a signal line that returns from the contact – hence it’s a two-wire system. Electronic contacts are actuated by coils, such as in relays; the coil itself requires 2 power wires, and the contacts it controls need another 2 wires, so in total there are 4 wires. Active contacts do not require a separate power supply; only one signal wire is needed. The so-called two-wire connection means that the other party supplies power to one end of the contact, while you connect the signal wire from the other end of the contact (the contact functions like a switch; in fact, what is referred to as a signal in DI terms is the power supply that passes through the contact and then returns). There are also some others such as proximity switches and photoelectric switches, which require two wires for power supply and one wire for the signal; they operate on a 3-wire system, with the power source provided by the device itself. It’s difficult to distinguish between active and passive types, as both types of contacts need electricity in order to function. In my opinion, so-called passive contacts are those whose power supply is provided by the controller himself, while the power for active contacts comes from other devices. The measuring instrument operates with analog signals; it can be four-wire, three-wire, or two-wire, and this is independent of whether active or passive contacts are used. In the four-wire system, there are 2 wires for powering the instrument and 2 wires for transmitting the signal. In three-wire systems, such as with older type of transmitters, the power supply for the instrument requires two wires carrying 24V DC; there is also one signal wire, and the negative terminal of the signal is connected to the 24V ground wire at the local site. The two-wire system is used in today’s transmitters; it involves one power line at 24VDC for supply, another line for the signal positive terminal, while the signal negative terminal is connected to a 24V ground line locally.