Wednesday, March 14, 2012

Inspection on electrical panels

Control and relay panels, Power Control centres and Motor control centres are the most commonly used Electrical panels. The main function of these panels is to provide the necessary protection to various equipment in the electrical circuits. Providing protection to electrical equipment like Transformers, feeders, cables, transmission lines are having great importance in electrical engineering. Protection is mainly achieved through circuit breakers. Circuit breakers are actuated with the help of various kinds of relays. Relays are an equipment that senses the parameters that is not desirable in the circuit like over current, over voltage or under voltage, over or under frequency, earth faults and so many other faults that may damage the electrical equipment. Basic function of a relay is to operate at a pre-set value of current, voltage, frequency or whatever function assigned to it.

Here we discuss about carrying out inspection of these electrical panels. The most common checks/tests to be carried out during the inspection are

1) Visual Examination

2) Dimensional Checks

3) Verification of panels as per the General arrangement Drawings (GAD)

3) Verification of components as per Bill of materials (BOM)

4) Checking the wiring continuity

5) Verification of functions of components and interlocks as per the schematic diagram

6) Insulation resistance measurement before and after high voltage test

7) High voltage (HV) test at power frequency

An inspection on these types of panels, basically to ensure proper functioning of all the components in the panel. A panel necessarily contains several components from switches to relays to maters to contactors to push-buttons. All these components are connected in the panels with wire according to a scheme. A scheme is essential to co-ordinate various functions as per the system requirement. An inspector ensures the correctness of the scheme during  the inspection.

To begin with the inspection of the panel, the first thing that needs to be done is to have a thourough visual inspection. This is to ensure proper placement of components, tightnes of connections, workmanship, proper termination of all wires and providing legends on all individual components. With the help of the Genaral arranement, the correctness of the arrangement of components, panel dimensions, panel paint shade, if anything specifed by the client can be verified. Dimensions of the panel as specified in the Genaral arrangement drawing must be measured during inspection and tallied. Major dimensional requirements are the width, depth, heightof the enclosures and base channel dimensions. Thickness of the panel enclure will also be available in the GAD that needs to be verified

Now, after the visual and dimensiona verification, the next thing to do is to check the correctness of components used in the panel. Every panel will have a Bill of materials (BOM) approved by the buyer. In BOM, specification of each component like its ratings, type number and make of the components wil be mentioned. The panel manufacturer is expected to use components stricly as per the BOM. 

Once the BOM is verified, the next step is to ensure correctness of wiring. Wiring shall be as per the schematic diagram approved by the buyer. For this, continuity between different points as mentioned in the drawing shall be checked using a continuity tester.

Once the continuity test is finished, the panel can be electrically activated with its rated voltage. All power circuits that directly supply power to various equipment shall be given the rated voltage and control circuit shall be given its rated voltage separately. After switching on, all the interlocks between various operations of the components shall be verified. Several interlocks are provided in the panel to ensure desired functioning of equipment whenever it is required to do so. for example, a relay picks when there is an over current in the circuit. This in turn trips the circuit breaker. Along with the relay that caused the operation of the circuit breaker, another auxiliary relay also trips that also needs to be re-set after attending the fault in the circuit.

Next step is to ensure the di-electric strength of the circuits and the components in the panel. All the terminals where the wires are terminated must be shorted together with the use of bare conductor wire. Power and control circuit shall be separately shorted. Continuity between each point after shorting shall be ensured. After this, insulation resistance shall be checked using an insulation tester. A value above 1 mega ohm is usually considered acceptable though a higher value is desirable for new panels. Subsequently a high voltage at power frequency (50 Hz in India) is applied across the entire circuit and the earthing point. For LT control panels, 3 Kv is applied across each phase and the earth point and for control circuit, 2 KV between the two supply points and the earth point. There shall not be any breakdown of the voltage during the test

The panel can be considered meeting the buyer's requirements if all the checks/tests mentioned above are satisfactory

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