In the long-term actual operation, the 24vdc relay is mostly caused by various faults due to damage to the contacts. The damage of the contact comes from two points: the temperature rise caused by the contact resistance during operation and the arc burn during opening and closing. According to the load capacity and the nature of the load, it can be divided into resistive load, inductive load, capacitive load and so on.
 
In electrical control devices, small intermediate relays are commonly used devices, and the current carrying current required by its contacts is generally not very large, and the resistive load and inductive load may not be too different. Moreover, the contacts of the intermediate relay are general. All have 5A capacity, and many have 10A contact capacity, and even have contact capacities as large as 60A and 80A. Even the 5A contact capacity is far greater than the operating current of most devices in the control circuit. So, in order to operate reliably, I think; in the design and installation of the control device, if it is a very large-capacity AC contactor's attracting coil control circuit, all devices with small contact capacity such as time relays, electrical contact pressure Meters, electric contact thermometers, travel switches, micro switches, proximity switches, transfer switches, control buttons, etc., all pass through a 24vdc relay with a capacity of 10A or more to control the AC contactor.

If the commonly used intermediate relay is an AC intermediate relay, it is usually a 220VAC intermediate relay, if it is a DC intermediate relay, then it is usually a 24vdc relay.