5. Electrostatic effects
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Making the producation line safe P.26
Aerosol cans are often filled with flammable liquids. Therefore, it is essential to reduce the chance of sparks on the production line. There are two ways in which sparks might be generated:
  1. a build up of charge on a capacitor
  2. an inductance (such as in a motor).

Let's see how we can reduce the chances of these causing sparks.

1. Stopping charge build up
The conveyer belt that runs through the production line can become charged as it rubs against its mountings. All of these mountings are earthed so they won’t get charged up. However, if the electrical potential on the top of the belt were to reach only a few thousand volts, then a spark might jump to the nearest earthed equipment. Although there shouldn’t be any flammable vapours nearby, this could still be hazardous. Therefore, it is important to keep the belt discharged.
Picture 5.15 As the motor spins, there are sparks at the commutator. These occur whenever the circuit is broken (i.e. the contact between the commutator and the brushes breaks).
Keeping the belt discharged
We have seen (page 24) that we can discharge a capacitor (like the belt) through a resistor. The time it takes to discharge is measured by the time constant, which is equal to the product of the resistance and capacitance.

To reduce the chance of sparking, we need to keep the resistance of the belt to a minimum. So the belts are made of a special rubber that is impregnated with carbon. This means that the belts remain flexible but are quite good electrical conductors. So, even though they are rubbing against the mountings, they won’t charge up because any charge can be conducted safely away through the belts.

2. Inductive sparking
You may have noticed that when you switch off a light, there is a small spark in the switch. This is due to the inductance of the lighting circuit. When the circuit is broken, the inductance tries to keep the current flowing and produces a large voltage across the switch. The voltage is enough to cause a spark.

The same effect will cause sparks at the brushes of a normal electric motor. As the motor is turning, the circuit is constantly broken (and reconnected) as the commutator and brushes swap the connections over. Every time it is broken, there will be a spark. The motors in a gassing room need to be made explosion proof.

Earth connection
Picture 5.16 The earth wire is bonded to the ground.
Earthing

You will often hear of electrical connections being connected to earth, earthed or grounded. This is, literally, making a good connection with the Earth. Often, at some point an earth connection is bonded to a metal pole that is driven into the ground. The reason that this is effective is that the Earth itself doesn’t really charge up. So its electrical potential is constant – often taken as zero. There are two reasons its potential doesn’t change:

  • it has such a big capacitance that any charge that builds up has no noticeable effect
  • on average, there is as much charge flowing onto it as off it.
Explosion proof motors
Motors (and other electrical equipment) are made explosion proof by sealing them in a special casing. The casing has to be strong enough to withstand any combustion of gases that get inside it. And it has to prevent the transmission of the explosion to the surrounding atmosphere. This would happen if hot gases escaped after they had been ignited, setting the surrounding gases on fire.

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Question 21
Circuits that carry large currents are often switched using a relay. The actual contacts of the relay are hermetically sealed (sealed in a vacuum or inert gas and not exposed to the air). The switch used by the operator switches on the current in the coil of the relay – and this closes the switch for the bigger current.

a) Why is this safer for the operator?


Click shift/return to get a line break in your answer

b) How does this improve the lifetime of the switch?

c) Would it be explosion proof?

Summary                                           Close
  • production line belts have a capacitance
  • they are made from special conducting rubber to help them discharge quickly
  • inductive circuits cause sparking when the circuit is broken
  • electric motors have to be enclosed in special explosion proof casing