1, choose
1 Select the appropriate voltage or current transformer according to the voltage level of the measured line and the current level to ensure the safety of the operator and the instrument.
2 Generally, the accuracy level of the transformer is twice as high as that of the measuring instrument. For example, a 0.5-level instrument must use a 0.1-level transformer.
3 Select the transformer of the appropriate rated capacity according to the size and nature of the load (including measuring instruments and connecting wires) that need to be connected to the transformer.
For example, a current transformer with a rated capacity S of 5 VA, at the secondary rated current I2N=5A and the power factor 
Maximum load that the secondary can access .
2, use
1 The secondary coil of the voltage transformer must not be short-circuited. The secondary coil of the current transformer is not allowed to open.
2 The secondary coil, iron core and outer casing of the transformer should be reliably grounded to ensure personal and equipment safety.
3 In addition to specially designed reversible transformers, general transformers are not allowed to be used in the opposite direction (ie, primary and secondary cannot be interchanged).
4 After the meter is connected to the transformer, the voltmeter reading should be multiplied by the transformation ratio. The current meter reading multiplied by the variable current ratio is the value of the measured high voltage and large current.
POE (Power Over Ethernet) refers to the use of IP-based terminals (such as IP phones, wireless LAN access points AP, network cameras) without any changes to the existing Ethernet Cat.5 cabling infrastructure. Etc.) The technology that can provide DC power for such devices while transmitting data signals is a switch that supports Power over Ethernet.
POE is also known as a power supply system based on local area network (POL, Power over LAN) or Active Ethernet (Active Ethernet), sometimes also referred to as Power over Ethernet for short. This is the use of existing standard Ethernet transmission cables to transmit data and data at the same time. The latest standard specification of electric power, and maintains compatibility with existing Ethernet systems and users. The IEEE 802.3af standard is a new standard based on the POE of the Ethernet power supply system. It adds related standards for direct power supply through the network cable on the basis of IEEE 802.3. It is an extension of the existing Ethernet standard and the first international standard for power distribution. standard.
IEEE began to develop the standard in 1999, and the earliest participating vendors were 3Com, Intel, PowerDsine, Nortel, Mitel and National Semiconductor. However, the shortcomings of this standard have been restricting the expansion of the market. Until June 2003, the IEEE approved the 802.3af standard, which clearly stipulates the power detection and control items in the remote system, and connects routers, switches, and hubs to IP phones, security systems, and wireless LANs through Ethernet cables. The method of power supply for points and other equipment is regulated. The development of IEEE 802.3af includes the efforts of many company experts, which also allows the standard to be tested in all aspects.
The IEEE 802.3at standard was formulated in 2005 and promulgated in 2009. The 802.3at standard output power can reach 30W, and the available power at the receiving end is 25.5W.
The IEEE 802.3 working group and various manufacturer alliances launched POH-POWER OVER HDBASET in the industry at the end of 2012.
Using the current 4-Pair four-pair wire technology plus, the bilateral power supply can reach 60-100W power, which can be achieved by using 5 or 6 types of wires. It is used in high-definition video promotion to supply power to 42" LED TVs. According to the new regulations in 2010, the power consumption of 42" LED TVs needs to be less than 70W.
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