Types of Medium Sizes of Laboratory

Types of Medium Sizes of Laboratory

Layout

1. The layout of a h.v. laboratory is an important aspect for providing an efficient testing facility. 
2. Laboratory arrangements differ very much from single equipment to multiple d.c., a.c., and impulse arrangements in different testing program.
3. Each laboratory has to be designed individually considering the type of equipment to be tested, and available space, other accessories needed for the tests, the storage space required, earthing, control gear, and the safety precautions require most careful consideration.

Laboratory Building

1. The building construction is not critical except where ionization tests are conducted.
2. To minimize the floor loading problems and to simplify earthing arrangement, ground level location is preferred. The floor should withstand the loading imposed I the equipment and test objects.
3. Arrangements should be made to ensure that the laboratory is free from dust, draught, and excessive humidity.
4. Laboratory windows may require blackout arrangement for visual corona tests, etc.
5. The control should be located in such a way as to include good overall view of the laboratory and test area.
6. The main access door to the test area must accommodate the test equipment.
7. The test object should have adequate interlocking arrangements and warning system to ensure safety to the personnel.

Safety

1. Safety is one of the most important requirements in a high voltage laboratory.
2. Proper operation of the HV Lab include the presence of qualified personnel, the use of systematic procedures, safety ropes, signs, visual and audible warnings and alarms, the use of grounding rods, the periodic review of safety rules, the availability of fire extinguishers, emergency phone numbers, first aid kit, and smoke alarm system.

Grounding, Earthing, Fencing, and Shielding of Test Set Up

1. These are intended to prevent risk to apparatus, installations and persons.
2. It is also required to avoid undesirable mutual interference between experimental set up and environment and to measure rapidly varying quantities without any disturbance.

Earthing

1. Complete earthing of walls, ceiling, floor and equipments are very essential.
2. Visible metallic connections with earth must be provided to each equipments at 2-3 points along with earthed cage or metallic fences with floor and celling.
3. Before touching the equipments earthing can be done with the help of insulated rods.
4. Introduced through the fencing mesh which is connected to ground connection inside.
5. Automatic earthing switches can be provided. In high voltage set ups with direct supply from high voltage network earthing can be achieved by earthing isolators.
6. Earthing should only follow after switching the current source off and should be removed only when there is no longer anyone present within the fence or if the set up is vacated after removal of earth.
7. All metallic parts of set up which do not carry potential should also be earthed with adequate cross section of at least 1.5 mm² Copper.
8. Dynamic forces which are introduced should also be considered for providing earth connections. 
9. The rapid voltage and current variations can occur during high voltage experiments due to breakdown process which can produce transient currents in earth connections causing potential difference may be equal to applied voltages.
10. The sudden voltage collapse on breakdown discharge occurs in short times so even lighting impulse voltage can appear. All these facts are required to be considered for providing safety measures like earthing and fencing.
11. Floor should cover plane earth electrode with high conductivity foil or closely meshed copper grid. Equipments earth should be connected to it by wire copper bands to keep voltage drop low or minimum.

Fencing

1. All high voltage set ups must be protected against unintentional entry to the danger zone which can be done with the help of fences.
2. All desired clearances as per IS should be kept depending upon the voltage range.
3. Fence should be earthed at more than one point and provided with danger board . Simple barrier chains can be considered where observance can be constantly supervised.
4. Entrances to the danger zone should be provided with locks which effect automatic switch off. 
5. An electric field develops between the elements at high voltage potential and the neighboring elements at earth potential.
6. When these stray capacitances discharges through earth it can produce transient currents and may cause over voltages partly outside the test object.
7. Therefore entire high voltage circuit should be surrounded by closed metallic shield like Faraday cage then only earth connections outside the cage remain current free.

Shielding

1. Electromagnetic waves are generated during breakdown discharge processes in high voltage circuits which can disturb surroundings.
2. Partial discharge measurements can disturb when high voltage circuit behaves as an antenna and receives external electromagnetic waves.
3. The disturbing influence of the surroundings on sensitive high voltage measurements is generally more intense than that exerted in turn by the high voltage investigations on the surroundings. 
4. The complete elimination of external interferences and at the same time of eventual environmental influences is achieved by using unknown metallic shielding.
5. High damping of electromagnetic fields is the objective in plane conductors intended for shielding purposes.
6. Therefore the closed meshed metal net is hung on or set into the walls of the laboratory and the unavoidable apertures for power and communications leads are blocked for high frequency currents with low pass filters.

Figure A

Figure B