Week 4 Flashcards
Define electric shock
Is the pathophysiological effect of an electric current flow through the human body
The current affects on human body (4) :
1- muscular
2- circulatory
3- respiratory function
4- serious burns
The degree of danger is a function of (3) :
1- the magnitude of the current
2- the part of the body where the current passes
3- the duration of the current
What is the maximum current to pass in human body?
Near the heart
30 mA
If near the heart the human is in serious danger if the current is not interrupted in a very short time
What are the causes of electrical fires? (4)
1- overload
2- short circuit
3- earth leakage current
4- electric arcs in cables and connections
Define the 4 zones
1- Imperceptible (0.5 mA)
2- Perceptible (200 mA)
3- Reversible effect : muscular contraction (500 mA)
4 zone :
4.1- up to 5% of heart failure (1000 mA)
4.2 - up to 50% heart failure (2000 mA)
4.3 - more than 50% probability of heart failure
Kinds of dangerous contact
1- contact with live parts
2- contact with another conductive parts under fault conditions
Corresponding protective measures
1- Basic protection
2- fault protection
Basic protection (4) :
1- protection by insulating live parts
2- protection by barriers and enclosure (widespread exmp: cabinets, assemblies, control panels, distribution boards)
3- obstacles & out of arm’s reach (only skilled or instructed persons have access)
4- use ELV (extra low voltage) or limiting the energy discharge (only for low power circuits)
Fault protection (2) :
1- earthing for all exposed conductive parts
2- automatic disconnection of the supply
How much voltage can the human tolerance ? AC and DC
50 AC
120 DC
Additional protection
High sensitivity RCD
Residual current operating device, which operates at 30 mA or less.
Highly sensitive fast tripping devices, detects residual current to earth (can be human or animal or anything else) then disconnects the power supply automatically to prevent danger.
Reasons where protection infallible (5):
1- lack of proper maintenance
2- imprudence or carelessness
3- wear or tear of the insulation
4- accident contact
5- situations where the insulation is not effective (like immersion in water)
Explain the work of the RCD
Toroidal transformer contains three coils , the primary coil contains line current and the secondary contains neutral current, they produce equal but opposite flux, in case of fault and current changes it will produce out of balance flux and current will be flowing in the third coil (sensing coil) which is connected to a relay
Skipped test circuit rcd
RCD must be provided for (3):
1 - circuits supplying general use socket outlet with rated current <= 32A
2- circuits supplying mobile equipment with current <= 32A
3- AC final circuit supplying luminaries
What is recommended about RCD?
It’s recommended to limit the number of socket out lets protected by high sensitivity RCD
What are the principles of TT system (4)
1- all extraneous and exposed partst connected to a common earth electrode
2- neutral point of the power supply earthed in a point outside the influenced area of installation
3- the impedance of the earth fault loop consists of two earth electrodes (for the supply and installation)
4. RCD:
In= 50 / Ra
Current rated in RCD and resistance of earth electrode for installation
For temporary supplies (to work sites,…) and agricultural and horticultural premises, the value of 50 V is replaced by 25 v
Selectivity is possible for four levels:
- Main general distribution board
2- local general distribution board - Sub-distribution board
4- socket outlets for individual appliances
What is selectivity?
The selection of the devices so that the nearest device for the fault will operate rather than upstream devices. To ensure that the fault will be isolated and the supply is maintained to other parts without distribution
Condition for selectivity (2):
I- ratio of residual currents must be >3
2- time delving the upstream RCD
