The intensity of the electric current is expressed in amperes (A). It represents the flow of electrons.
Each material is characterized by its resistance in ohms. That is to say, the resistance it opposes to the passage of the electron current.
A material with resistance R, under a voltage U, allows an intensity (an electron current) to pass through I = U/R
Electrical power is directly linked: to the flow of electrons (current) and to the voltage of the generator (in a way: “the strength of the electrons”). It is expressed by the relationship: Power = Electron flow x “Electron strength” In other words: P = I x U or P = U x I in Watt If we know 2 of the 3 quantities of an apparatus, we can determine the 3rd one.
Tension domain
- TBT (Very Low Voltage)
- BT (Low voltage)
- HTA (High Voltage A)
- HTB (High Voltage B)
As an alternative
- U≤ 50 V
- 50U≤1,000V
- 1,000U≤50 KG
- U50 KG
Continuously
- U≤ 120 V
- 120U≤1500V
- 1500U≤75 KG
- U75 KG
The voltage ranges are wider continuously because the current is less dangerous at the same voltage. The nervous system controls the muscles by electrical impulses (max 1V). Each alternation of the alternating voltage is considered to be an order of contraction by the muscles. A continuous voltage causes much fewer muscle contractions once the current is established. On the other hand, the risks of internal burns and chemical blood reactions are the same.
0: non-insulated device
I: device whose masses must be connected to ground
II: device protected by double insulation (not connected to ground)
III: device powered by 12V or 24V
The Neutral Regime defines how:
- the ground of the voltage source (e.g. an EDF distribution transformer, a generator, a wind turbine,...)
- and the masses on the user side. That is, the way in which the metal carcasses of the devices are connected.
The first letter indicates the state of NEUTRAL with respect to earth:
- T: neutral to earth
- I: neutral isolated from ground
The second letter indicates the condition of the MASSES:
- T: masses on the ground
- N: neutral masses
There are several neutral regimes, the best known of which are:
The neutral TT regime:
- The first T indicates that the neutral of the installation is connected to ground on the generator side
- and the second indicates that the masses (metal frame) are connected to ground
The TN neutral regime:
- The first letter “T” indicates that the neutral of the installation is connected to ground on the generator side
- and the “N” indicates that the masses (metal frame) are connected to the neutral
The IT neutral regime:
- The first letter “I” indicates that the neutral of the installation is isolated from ground (So no connection) on the generator side
- and the second indicates that the masses (metal frame) are connected to ground
The TT diet
This pattern is the most classical, the most widespread (especially in the home).
Wiring:
- The neutral is grounded (at the transformer)
- All the masses are on the ground
In case of defect (escape of a phase on the carcass):
- The frame being connected to ground, the current returns to the transformer through it.
- The differential detects the current leak and cuts off the power.
The TN diet
Wiring:
- The neutral is grounded (at the transformer)
- The masses are in neutral
In fact, this diet is very similar to the classic TT diet, but:
- The neutral and ground wires are one and the same wire (which prevents the use of the differential)
- We will never cut the neutral (especially at the level of the table), as the earth is not differentiated.
In the event of a defect (a phase affecting the carcass for example):
- A short circuit is created between phase and neutral.
- It causes the circuit breaker to break ONLY if the fault is fairly obvious.
The IT regime
It is only found in France, in systems where operational continuity is important and with continuous maintenance.
Wiring:
- The neutral is not connected to ground, it is isolated from ground.
- The masses are connected to the ground.
In case of defect:
- The fault point is fixed to the earth's potential, in other words, the fault sets the potential of the relevant point of the installation to 0V
- Since there is no potential difference (0-0 = 0 Volt), there is no leak current and therefore no danger.
- Since the fault does not trigger any interruption, the continuity of operation is ensured
- The faulty phase being fixed to the ground potential, i.e. 0 volt: the 2 other phases will have a potential of 400 Volt with respect to earth, and the neutral one will have a potential of 230 Volt with respect to earth
If the fault is introduced by an operator who touches a phase, the principle is the same (it is the operator's hand that will fix the point at the system's potential 0). No danger.
If a second defect occurs:
- The first phase is always set to Earth's potential, i.e. 0 Volts.
- Grounding the 2nd point (phase or neutral) will cause a short circuit
- The circuit breaker will cut in overcurrent or short circuit
If an operator is causing the defect:
- It will experience a potential difference of 380 Volt (phase) or 220 Volt (neutral)
- It will not be a good enough conductor to cause an overcurrent or short circuit break
- No protection will be triggered
We can see that the system is very protective in the case of a single defect, but becomes lethal at the second defect:
- It is therefore necessary to detect the first defect
- and to urgently repair it
Before the second life-threatening defect occurs.
In the IT regime, the system will therefore be equipped with an isolation controller who will inform of the defect (visual and/or acoustic alarm)
The correction of the defect is imperative and urgent.
Safety standards therefore require the permanent availability of qualified maintenance personnel on site.
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