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Danger! - Labooratory Safety Officer
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Chemical plants, oil refineries, gas works and mines are all places where there
is a risk that vapours may be present. Many of these vapours may be flammable:
they may burn or explode under certain conditions. In an oil refinery this might
be petroleum vapour, down a mine it may be methane: both, in the right mixture with
air, may explode.
In the 19th Century, miners used oil lamps and candles to light their way. The heat
from these lamps and candles could cause the methane to explode, and this led to
several serious accidents with great loss of life. Sir Humphry Davy invented a lamp
that did not cause methane-air mixtures to explode. The principle that he used is
still used today in flame-arrestors.
Nowadays, most apparatus for lighting and machinery is electric, and we must also
make sure that this apparatus cannot ignite these flammable vapours. The spark from
simply switching on an ordinary, battery-operated torch is enough to cause a gas
mixture to explode.
Fortunately, a great deal of electrical apparatus can be made safe. Low-voltage
equipment, such as telephones and bells, can be made safe by a method called
'intrinsic safety'. A signalling bell is used to demonstrate this.
In an ordinary electric bell an electromagnet is switched on and off to pull the
hammer to the bell and let it spring back - see figure 1. Every time that the
current to the electromagnet switches off, the magnetic field in the coil falls.
This causes a large potential difference across the ends of the coil. This may
cause a spark to appear at the contacts, and this could ignite an explosive
atmosphere.
The solution to this problem is to prevent reverse currents in the coil. This is
achieved by putting in a bridge rectifier - see figure 2. Charge can only flow in
the direction of the arrows, which is the direction of the current, which creates
the magnetic field. With no reverse current, no spark can be produced. The bell
can be used safely in an explosive atmosphere, without the risk of it causing an
explosion.
High-power equipment presents a bigger challenge, but this can be put inside a
heavy metal box. On the one hand, the box will prevent sparks from igniting the
atmosphere outside the box; on the other hand, the box can be made strong enough
to cope with an explosion inside it. The box cannot be made airtight, because
electrical apparatus tends to get hot, so there needs to be a way for heat to
escape from the box. To allow any heat to escape, the lid of the box has a small
air gap between it and the body of the box. This air gap must be precisely long
enough and wide enough to prevent any flame emerging from the box.
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Figure 1: An ordinary electric bell
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Figure 2: An electric bell with a bridge rectifier
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