干触点和湿触点区别
干接点(Dry Contact)，相对于湿接点而言，也被称之为干触点，是一种无源开关，具有闭合和断开的两种状态，两个接触点之间没有极性，可以互换;
常见的干接点信号有:
1，各种开关，如限位开关、行程开关、脚踏开关、旋转开关、温度开关、液位开关等;
2，各种按键;
3，各种传感器的输出，如:环境动力监控中的传感器:水浸传感器、火灾报警传感器、玻璃破碎、振动、烟雾和凝结传感器;
4，继电器、干簧管的输出;
湿接点(Wet Contact)，相对于干接点而言，也被称之为湿触点，是一种有源开关，具有有电和无电的两种状态，两个接点之间有极性，不能反接。
常见的湿接点信号有:
1，如果把以上的干接点信号，接上电源，再跟电源的另外一极，作为输出，就是湿接点信号;工业控制上，常用的湿接点的电压范围是DC0,30V，比较标准的是DC24V;AC110,220V的输出也可以是湿接点，尽管这样做比较少;
2，把TTL电平输出作为湿接点，也未尝不可;一般情况下，TTL电平需要带缓冲输出的，例如:7407、245、244等，与VCC等构成回路;244、245也可以跟gnd构成回路;才能驱动远方的光耦。
3，NPN三极管的集电极输出和VCC;
4，达林顿管的集电极输出和VCC;
5，红外反射传感器和对射传感器的输出。
在工业控制领域中，采用干接点要远远多于湿接点，这是因为干接点没有极性带来的优点:
随便接入，降低工程成本和工程人员要求，提高工程速度
处理干接点开关量数量多
连接干接点的导线即使长期短路既不会损坏本地的控制设备，也不会损坏远方的设备
接入容易，接口容易统一
干接点和湿接点的调理方法:
采用光耦的光电隔离。
干接点的接入光耦，顺序如下:
VCC?限流电阻?光耦LED?干接点1?干接点2?GND
也可以:
VCC?干接点1?干接点2?限流电阻?光耦LED?GND
湿接点的接入光耦，顺序如下:
湿接点1?限流电阻?光耦LED?湿接点2
湿接点如果是DC电源，如果湿接点1和2反了，显然打不开光耦;
湿接点如果是AC电源，可以相反。
Volt-free and dry contact mean the same thing. If a control system supplier offers a dry contact for you to read as a status bit, then he is offering to close a contact (relay or contact output) that is nothing more that a stand-alone set of contacts with no voltage, current, or anything else impressed across the contact set. It becomes the user's responsibility to determine how to sense that contact closure. Usually you do this by putting a voltage on one side and sensing the voltage on a return line from the other side of the contact when it closes. Once you apply voltage to the contacts, it becomes a wet contact. You "wet" the contact with a sensible voltage level.
A proximity contact is a contact set that makes without any kind of direct coil or physical actuation of the contact set. Proximity contacts or switches are usually either optical devices that switch because a light beam is broken or completed, or in some cases magnetic switches are called proximity contacts. The contact set will open or close when a magnet gets close enough to cause the switch action like a door switch on a burglar alarm
I think the guys above talk about Relays, and not about Current Transformers as per your question.
Assume you want to measure the current in a circuitry. there are 2 standard ways a) you put an Ampere Meter within the circuit
b) you put a small resistance within the circuit, and measure the voltage across that resistor.
Both methods mean that you have the circuit's voltage standing on your measurement device.
With a Current Transformer, you can avoid that. You have just a few (say 3 - 5) windings of thick wire on the primary side which goes in series in your circuit, and you have a good number of thin wire windings on the secondary side, with a voltage generated by the magnetic field of the current transformer, and which can be measured as a function of the current in the circuit.
So you measure a circuit current with a "Volt Free Contact".
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