1.4 Electricity and safety

• Natuurkunde
• Middelbare school
• havo
• Leerjaar 3

1.4 Electricity & safety 

Quantity	Symbol	Unit	Symbol
	U
Current
Resistance

Volt

I

Ohm

R

Voltage

Ω

Ampère

V

A

Most of the fuse groups in our home consumer unit (meterkast) can handle max current of

1. 3A
2. 1A
3. 16A
4. 230V

The following appliances connected to one group of the home installation are all working at the same time:
– a 1200 W fridge - a 1400W freezer
– a 200 W extractor hoof - a 50W bulb
The group has a 16 A fuse.
1 The total power is 2850W
2 The fuse will trip

1. Only 1 is true
2. Only 2 is true
3. 1 and 2 are wrong
4. 1 and 2 are true

Lesson goals:

Leg uit wat de volgende woorde betekenen:

• I can distinguish between contact resistance and bodily resistance
• I can distinguish between single and double insulation
• I know what a fuse is
• I know what an earth is and how a earth leakage works

DANGERS:

• When there is a high current 

       passing through a conductor
       they get so hot that it can
       cause a fire.

• When touching a conducting 

       material you experience an

       electric shock.

Current through your body

- When the current passing  

   through your body is small, you 

   maintain controll over your 

   muscles. 

- If the current through your body 

   is too big then you loose control 

   and your muscles can not relax.

Electrical resistance of your body and contact resitance

The magnitude of the current depends on two things:

1 - How great the voltage is

2 - How great the resistance of the body  is

The total resistance is the sum of these two

I=​R​​V​​

Dry skin = great contact resistance      wet skin =low contact resistance

Single and double insulation

• The conducting wires in a  household elecrrical system have been selected to allow 16 A current to pass through it. 
  
• The conducting wire has been insulated with pvc.
  
• This prevents you from getting a shock or a short circuit to occur.
  
• Some apparatus have double insulation

Safety preacuations in the metercabinet      

     FUSE

- Each circuit has its own circuitbreaker or circuit fuse

- If the current is greater than  

   16 A the circuitbreaker will flip

   switching off the current.

- This prevents a fire from starting.

Earth leakage

- The earth leakage compares the current in the phase wire and the neutral wire.

- If they are the same size there is no problem and the earth leakage will allow the current to pass through.

- If the difference in current between the
phase and the neutral wire is more than 30 mA, 

the earth leakage flips.

Earth

- This prevents the leakage current from passing through someones body.

- Therefore a green yellow earth runs from the outer casting of an apparatus back to the socket

and then to the earth rail in the

meter cabinet.