TT = 200 Ω (21 Ω)

TNS = 0.8 Ω
TNCS = 0.35 Ω

T=Earth (terra)

N=Neutral

C=Combined

S=Seperate

 

Protective Devices

When a fault occurs we need the C.P.D to switch off

Fuses and MCB's only know how much current is flowing

 

So we need as much current as possible when there is a fault to get fast disconnection times. 

An earth fault is a connection between a live conductor and extranous metal

a short circuit is contact between two live conductors the current goes up the live conductor and goes back down. 

Fast disconnection times

              ^

              |

Lots of current 

             ^

             |

Low impedance

 

pen = protective, earth, neutral

 

 

Types of fault

 <1.overload

   |Too much load, more current than is safe for the circuit

   |typically a few extra amps up to about 3x rating

 <2.short circuit

 <contact between live conductors

   |1Ø l-n

   |3Øl-l (400v)

   |typically 1000s of amps

 <3.earth fault

Contact between a live conductor and anything else. 

 

Fuses - Thermal effect

---------- Fuse element

 

HBC Fuses - High breaking capacity 

 

 

The rating I.e 6amps

10amps

16amps

32amps

etc

The rating is the lowest current that will break the device operate (switch off, break) 

Breaking capacity is the highest current the device can safely interupt. 

 

Why is sand in fuses?

It stops arcing as the fuse element melts

'Arc quenching'

 

30mA is the rcd average trip point.