Electricity in laboratory

A reliable energy supply should be available to ensure continuity of the work in a laboratory. The energy can be provided from the following sources:

— mains electricity supply

— generators

— solar energy supply system.

Remote laboratories often have problems in ensuring a continuous supply of elec- trical power and may need to generate electricity by using a local generator or a solar energy supply system.

1. Sources of electricity

Generators

Electrical energy can be provided by a fuel generator. It is possible to use the com- bustion engine of a motor car or a purpose-built generator. A purpose-built genera- tor produces an alternating current of 110 volts ( V) or 220 V and can usually generate more energy than a car engine. A car engine provides a direct current of 12 V or 24 V, which can be fed into rechargeable batteries (see below).

The type of current available will limit the selection of laboratory equipment; for example, an instrument that requires direct current can be supplied with energy from:

— batteries

— a direct current network with a transformer

— an alternating current network with a converter.

The installation of a direct current network is simple and it is safe to operate. However, for instruments that require a low-voltage (6 V, 12 V or 24 V) direct cur- rent, the high voltage produced from the direct current network must be converted by means of a transformer. Alternatively, for instruments that require alternating current (110 V, 220 V or 240 V ), the direct current must be converted into alternat- ing current by means of an inverter. Inverters are heavy and expensive and significant energy losses occur in the conversion process. It is therefore preferable to use either direct current or alternating current appliances, depending on your supply, and avoid the need for conversion.

If no generator is available or if a mains power supply is accessible, but the electri- cal current fluctuates or is prone to frequent breakdowns, a solar energy supply may be preferable (see below).

Solar energy supply systems (photovoltaic systems)

A laboratory with a few instruments with low energy requirements can work with a small energy supply. For laboratories located in remote areas, a solar energy supply system may be more suitable than a generator since there are no problems of fuel supplies and it can be easily maintained.

A solar energy supply system has three components:

— solar panel(s)

— an electronic charge regulator

— batteries.

Solar panels

Two different types of solar panel are commercially available:

— panels with cells of crystalline silicon

— panels with cells of amorphous silicon.

Amorphous silicon panels are less expensive, but produce solar energy less efficiently than crystalline silicon panels.

Solar panels must be installed so that they are exposed to direct light, since shade reduces the efficiency of energy production. They should be inclined at an angle of 15°. The underside of the panel must be freely ventilated. The minimum distance of the underside of the panel from the surface of the supporting construction must be more than 5 cm to avoid heating of the panel, which would reduce the efficiency of energy production.

Electronic charge regulators

A charge regulator controls the charging and discharging of the batteries automati- cally. When the battery voltage falls below a threshold value during discharge, the laboratory instrument will be disconnected from the battery. On the other hand, if the voltage increases above a threshold value (e.g. when the battery is recharged), the solar panel will be disconnected from the battery. A good charge regulator adapts the maximal voltage of the battery to the change in the temperature of the ambient environment. This prevents the loss of water in the battery by evaporation. It is important to keep a spare charge regulator in stock in case of breakdown. The charge regulator chosen should be stable under tropical conditions. It is advisable to choose a charge regulator with an integrated digital display that allows the bat- tery charge to be monitored easily.

Batteries

Lead batteries

Solar energy systems require rechargeable batteries, which may be either lead or nickel–cadmium (Ni–Cd) batteries. Lead batteries are preferred and many types are available commercially (see Table 2.1). High-efficiency batteries have practical advantages, although they are more expensive than normal batteries.

When purchasing batteries choose 12 V batteries with the highest capacity (1000 ampere-hours (Ah)).

Several types of maintenance-free lead batteries are commercially available, but they are expensive and less efficient than those that require maintenance. The de- velopment of this type of battery is still in progress; it has not been thoroughly tested in tropical climates. Therefore, the maintenance-free batteries are not rec- ommended.

Transport of lead batteries

Lead batteries should be emptied before being transported. It is important to re- member that if lead batteries are to be transported by air they must be empty of electrolyte solution, which should be replaced on arrival at the destination.

Table  2.1  Specifications for batteries used  for solar power supply

Maintenance of lead batteries

The daily discharge of lead batteries should not exceed 20% of the batteries’ ca- pacity, otherwise the lifetime of the batteries (normally about 1100 recharge cy- cles), will be shortened. If the batteries are repeatedly discharged to 40% of their capacity, they will last for only about 600 cycles. (There are some special lead batteries available that can be discharged by 40%, but will last for about 3000 recharge cycles.) For maintenance the level of fluid must be checked regularly and when necessary refilled with the distilled water that is used for car batteries.

High-efficiency batteries cannot be replaced by normal car batteries in case of a breakdown.When only car batteries are available to replace a defective high-efficiency battery, all the batteries in the energy storage system must be replaced with car batteries.

Nickel–cadmium (Ni–Cd) batteries

Ni–Cd batteries can be recharged by a solar panel. Some Ni–Cd batteries are the same size, but have different capacities.The AA-size Ni–Cd battery is available with a capacity from 0.5 Ah up to 0.7 Ah. Choose the batteries with the highest capacity. The small Ni–Cd batteries, type AAA to D, for use in laboratory instruments should be recharged in advance to enable continuous operation in a laboratory.The lifespan of Ni–Cd batteries may be 1000 recharging cycles, depending on their quality.

Maintenance of Ni–Cd batteries

Ni–Cd batteries appear to work unreliably in tropical countries. This apparent unreliability is caused by an increased rate of discharge rather than inefficient re- charging of the battery at high ambient temperatures (see below). Such problems may be partially overcome as follows:

● Ni–Cd batteries should be recharged at a low ambient temperature (e.g. in a refrigerator or in a specially constructed recharging box) shortly prior to being used. (For example, only 62% of the energy can be made available from a Ni–Cd battery that was charged at 40 °C.)

● Recharged Ni–Cd batteries should be stored under cool, dry conditions to mini- mize their rate of self-discharge. (For example, a Ni–Cd battery stored for 2 weeks at 40 °C will have a residual capacity of only 32%.) High humidity will also accelerate the self-discharge of the battery.

2. Setting up simple electrical equipment

If the laboratory has an electricity supply the following equipment can be used:

— an electric lamp for the microscope (stable illumination makes adjustment easier);

— an electric centrifuge (much faster than the manually operated type);

— a microhaematocrit centrifuge (for detection of anaemia);

— a spectrophotometer or colorimeter (allows accurate estimation of haemo- globin);

— a water-bath, refrigerator etc.

You may have to make simple connections or repairs to this equipment in the labo- ratory. The explanations given below are intended to help the laboratory technician to do this and are limited to the steps to follow in each case. Inexperienced persons should start by carrying out the procedures in the presence of an instructor.

Fig. 2.3 An electricity meter

The electricity meter (Fig. 2.3)

An electricity meter measures and records the amount of electricity used. It indicates:

— the voltage, measured in volts (220 V, 110 V, etc.);

— the strength of the current, measured in amperes (A);

— the frequency of the alternating current, e.g. 50 hertz(Hz) (cycles per second).

Some types of meter have switches or buttons:

— a flip-switch that can be flipped one way to cut off the electricity supply to the whole building (the mains fuse) and the other way to restore it;

— a button marked “OFF” that can be pushed to cut off the electricity supply;

— a button marked “ON” that can be pushed to restore the electricity supply.

The flip-switch or “OFF” button also acts as a circuit- breaker, automatically cutting off the current when the cir- cuit is overloaded.When this happens, first find and correct the fault that caused the cut-off, then press the “ON” but- ton or flip the switch to restore the current.

Setting up new electrical equipment

Voltage

Check that the voltage marked on the instrument is the same as that of your elec- tricity supply. The instrument has a label on it stating the voltage with which it must be used. The voltage of your electricity supply is marked on your electricity meter.

Dual-voltage equipment

Dual-voltage instruments can be used with two different voltage supplies.

There is a device on the instrument that enables you to select the appropriate voltage, i.e. the voltage marked on your electricity meter. Depending on the instru- ment, this device may be:

— a lever or switch that can be moved to the 110 V position or the 220 V posi- tion (Fig. 2.4(a));

— an unwired plug that can be transferred from the 110 V position to the 220 V position (Fig. 2.4(b));

— a screw that can be turned to the 110 V position or the 220 V position (Fig.2.4(c)).

Fig. 2.4 Dual-voltage instruments

The electrical power of the instrument

The electrical power is measured in watts (W) and is marked on the plate that shows the correct voltage for the instrument. Each piece of electrical equipment in the laboratory uses a certain amount of power. The total power used at any one time must not exceed the power of your electricity supply. You can work out how much power is available from the figures shown on the meter: multiply the voltage (V) by the current (A). For example, if the voltage is 220 V and the current is 30 A, the electrical power supplied will be 220 ¥ 30 = 6600 watts or 6.6 kW.

Using a transformer

If an instrument is intended for use with a voltage different from that of the labora- tory electricity supply, it can be used with a transformer. For example, if the centri- fuge provided only works at 110 V and the voltage of your electricity supply is 220 V, ask for a 110 –220 V transformer, indicating the wattage of the centrifuge. Plug the centrifuge into the 110 V connection of the transformer supplied, then plug the 220 V lead from the transformer into the laboratory electricity supply (wall socket).

Switching off electrical equipment

After an instrument has been switched off, it must be unplugged from the wall socket. If left plugged in, it is a fire risk.

3. What to do in case of failure of electrical equipment

If an instrument does not work, check the following:

— the fuses

— the plug at the end of the cable

— the cable

— the wall socket

— the voltage of the instrument and that of the electricity supply.

Before doing anything, cut off the electricity supply:

— either by pressing the button or the switch marked “OFF” on the meter

— or by removing the mains fuse (Fig. 2.5).

Tools (Fig. 2.6)

● Screwdriver

● Wire-cutters

● Flat-nose or taper-nose pliers

● Fuse wire

● Various spare parts: plugs, switches, etc.

Fig. 2.5 Removing the  mains  fuse

Fig. 2.6 Tools for electrical  work

Fig. 2.7 Removing fuse  wire  from a blown fuse

Fig. 2.8 Changing  a two-pin fuse

Fig. 2.9 A two-pin plug

Changing the fuse

Remove the cover from the fuse box.

If it is a screw-type fuse, the fuse wire is stretched between two screws. If the wire is broken or melted, the current no longer passes: the fuse has blown. Loosen the two screws (Fig. 2.7). Remove the old fuse wire. Replace it with new fuse wire of the same gauge (thickness), or with thinner wire if the same size is not available. Fix the wire in an “S” shape, with a loop at either end. The wire must pass beneath the small washers under the screws.

If it is a two-pin fuse, fix the fuse wire to the base of the pins, and then tighten the pins with pliers (Fig. 2.8).

Once the fuse has been repaired, check the whole circuit before switching on the electricity supply.

Checking the plug

If a fault is suspected in a plug, it must be repaired or replaced. There are many different types of plug; some have a screw on the outside that can be unscrewed so that the cover can be removed.

Two-pin plug (Fig. 2.9)

Inside the plug, the two wires of the cable are fixed to the terminal screws (T) of the contact pins (P). Check that the terminal screws are tightened. Sometimes this is all that is needed to repair the plug.

Fitting a new plug

To fit a new plug, remove the insulating material along a length of 1.0–1.5 cm from the end of each of the two wires making up the cable. This can be done by scraping with a knife but take care not to damage the wire inside. Twist the exposed ends of both wires to allow them to fit neatly into the terminal once the screw has been loosened (Fig. 2.10).

Insert one exposed end into each of the terminals of the plug.Tighten the terminal screws and replace the terminals (Fig. 2.11).The screws should hold the wires firmly; check by pulling the wires gently.

Fig. 2.10 Twist the  exposed ends  of both wires

Fig. 2.11 After inserting the  wires  into  the plug  terminals, tighten the terminal  screws

Three-pin plug (Fig. 2.12)

Two of the pins are connected to the electricity supply; one is “live” and one is “neutral”.The third (usually the middle) pin is connected to the “ground” or “earth”. It is most important to connect each of the three wires in the cable to the correct pin, and the plug usually contains instructions that should be strictly followed. If there is the slightest doubt, consult an electrician.

Fig. 2.12 A three-pin plug

The ground or earth wire is covered in green or green and yellow insulating material. It provides an escape for the electric current in case of poor insulation, thus avoid- ing passage of the current through the human body.

Checking the cable or switch

Check to see whether the cable is burned or broken. If so, it should be replaced. There are many different types of switch. They have to be unscrewed and opened if you want to check that they are working properly. Make sure that the two incoming wires and the two outgoing wires are firmly fixed in their respective terminals (Fig. 2.13).

Fig. 2.13 A switch

Extension lead

An extension lead is a cable with a male plug (M) on one end and a female plug (F) on the other (Fig. 2.14). The fe- male plug is fixed to the cable by two terminals inside the plug, just as in the normal male plug.

Checking the wall socket

To check a wall socket, plug in a lamp that you know to be working. Some sockets are fitted with a small replaceable fuse. If this is not the case, it is usually wise to call in an electrician to repair a wall socket.

Precautions

● Never take electrical equipment apart without first disconnecting the electricity supply.

● Never touch electrical equipment with wet hands (water is a good conductor of electricity).

● Never plug a new piece of equipment into the electricity supply without first checking the plate to see whether the voltage marked is the same as that of the laboratory supply (110 V, 220 V, etc.).

● Never remove a plug from a socket by pulling the cable.

● Never replace fuse wire with wire that is thicker.