Introduction
Any complete set of instrument and electrical control cabinet products must be tested and verified according to design requirements before leaving the factory.
I. Test Preparation:
1. All power switches must be turned off before powering on the PLC control cabinet or I/O cabinet. 2. All connections between the PLC or DCS card and external devices must be disconnected. For example, for a Siemens S7-300 PLC, all front connectors should be loosened and lifted. 3. Check that all power terminal wires and connections are securely crimped. Loose power connections can cause instruments or electrical appliances to burn out. 4. Check that the hardware settings of all instruments and electrical appliances are consistent with the design requirements; check that the power settings and circuit settings of the terminal blocks are consistent with the design requirements.
II. Powering on:
1. Principles:
Power should be supplied to each switch in stages, starting from the main power supply. Before supplying power, check whether the input voltage and output impedance of the switches are normal. If the output resistance is low, the cause should be analyzed and confirmed. Multiple instruments in the cabinet should not be powered on simultaneously; one instrument should be powered on normally before the next is connected. 2. Before closing the main power supply, first confirm whether the AC voltage level meets the requirements. For switches with two AC power inputs without automatic switching, strictly avoid parallel connections. For switches with automatic switching, test the dual-power automatic switching device under no-load to ensure it operates normally. 3. After the AC power supply is normal, supply power to the DC power supply, checking whether the DC power supply output voltage is normal and whether it is properly supplied to the input side of each DC switch.
4. Check that the DC power supply is connected to the terminals of all electrical equipment, including the power terminals of the PLC expansion module. After checking, unused instruments can be temporarily turned off to await the next test.
III. Signal Injection:
1. Analog Signal Testing: When injecting a signal into the instrument using a signal generator, the nature of the injection point and the output type of the signal generator must be clearly defined. Voltage signals are not allowed to be injected into the current signal input terminal. For the AI input terminal, the corresponding signal must be injected while energized according to the signal type (2-wire, 4-wire, voltage, resistance, mV, etc.). Selecting the wrong signal will cause the signal generator or instrument to burn out. Special attention should be paid to the difference between two-wire and 4-wire signals. When testing the AO signal, a current signal can be injected into the front of the card, and an ammeter can be used to directly short-circuit and measure the signal at the terminal.
2. Switch Signal Testing: For DC24V type DI or DO points, a DC24V signal can be directly injected. The injected signal must be connected to a fuse of 0.5A or less, including when simulating dry contact signals by shorting the DI input terminal; a fuse-protected jumper wire should also be used. When injecting signals into the instrument using a live device, the nature of the injection point must be clearly defined. Testing DI cards: When simulating level signal input, a DC signal can be directly injected into the level input terminal using a fuse-protected jumper wire. When simulating dry contacts, the input terminal is directly shorted using a fuse-protected jumper wire. Testing DO cards: A signal of the corresponding voltage can be injected into the front connector. Before injection, ensure the injection point is not energized and use a fuse-protected connecting wire. The front connector must be loosened and lifted out of the module.
IV. Precautions:
1. When the instrument or PLC is energized, it is not allowed to inject a current signal into the current signal output terminal. Consequence: The instrument will be burned out. 2. It is not allowed to use the resistance range to measure the resistance or continuity of energized equipment or components. Violation of this requirement will result in the resistance range being burned out. 3. The precision resistance box is a signal transmitter and can only be used as an analog resistance temperature signal source. It is not allowed to be used as a variable resistor for other tests. 4. When using an ammeter, it is generally connected in series in the circuit. If the ammeter is directly short-circuited to measure current, it is only allowed in the following situations: the four-wire current output terminal of the instrument, such as the current output terminal of the safety barrier, the output terminal of the PLC AO module, the current output terminal of the instrument adjustment signal; the signal input terminal of the PLC AI module that receives four-wire signals. When using the AI module 7KF02 of the Siemens PLC, if the range block is set to D (with a two-wire transmitter), it is not allowed to short-circuit the current measurement on the PLC terminal. 5. When connecting the ammeter in series in the circuit to measure current, ensure that the appropriate current range is selected.
V. Conclusion:
After completing the above tests, reconnect to the DCS and PLC cards, check the power supply of each card, and run the program for joint inspection testing. Design defects, setting errors, and incorrect wiring are inevitable during the assembly of the control cabinet. Following the above steps can effectively prevent accidents such as burning out PLC or DCS cards during the calibration process.
