Transcript Dia 1 - XAMK Moodle

PROTECTION AGAINST INDIRECT CONTACT
1.
Earthed equipotential bonding and automatic
disconnection of the supply
Over current protective devices:
*Miniature circuit breakers
* Fuses
*Residual current protective devices
2.
Using class II equipment or equivalent insulation
* Double insulation
3.
Electrical separation
* Isolation transformer
* Non - conducting location
4.
Using extra - low voltage
* U <= 50 V AC
1 Earthed Equipment
Class 1 electrical equipment is provided with one layer of insulation over the live conductors, and exposed metalwork
is bonded to earth so that it cannot become live in the event of an insulation failure. The external metal casing of any
item of electrical equipment must be earthed as a legal requirement. With correctly earthed supply installations and
equipment, the risk of electric shock is virtually nil.
2 Residual
Current Devices (RCD)
The application of a Residual Current Device (previously known as Earth Leakage Circuit Breaker) to a
conventionally earthed system should be considered where it is vital to provide an additional back-up protection
against failure of the primary earthing system.
As a general rule, an RCD will prevent a person from being subjected to a lethal shock from a fault current to earth by
limiting the magnitude of the shock to 30 mA and the duration of the shock to 30 ms. An RCD will give no protection
from a live to neutral contact.
3 Double
Insulated Equipment
Class 2 electrical equipment has all exposed metalwork separated from the conductors by two layers of insulation, so
that the metalwork cannot become live. There is no earth connection and the operator's safety depends upon the
integrity of the two layers of insulation.
Double insulated equipment is marked with the symbol:
and is using Euro socket :
Residual Current Device RCD
ELECTRIC APPLIANCE
RCD Socket
Fault current
The fault current, via the toroid, supplies energy to an electromagnet
whose moving part is held by a permanent magnet. When the
operating threshold is reached, the electromagnet counterbalances
the
attraction of the permanent magnet and the moving part, drawn by a
spring, opens the magnetic circuit and mechanically actuates circuitbreaker opening.
PE
N
230 V
L
A Residual Current Device (RCD) is a circuit protection device which disconnects the load ("trips") if the current flowing out in the live
wire exceeds the current returning in the neutral by a given amount. (Typically 30mA, 100mA, 300mA, depending on the device). 30mA =
0.03A
This imbalance in the return current (the residual current) is leaking to earth, which could indicate a fault in the circuit or a person
receiving an electric shock (to earth).
If an RCD is being used to provide additional protection against electric shock, the Wiring Regulations specify that a type with no time
delay & 30mA sensitivity be used. In most cases a shock of under 30mA or one for the brief period before the RCD trips will not be fatal.