In France, the field very low tension (abbreviation TBT ) is governed by the Décret n° 88-1056 of November 14th, 1988, which treats protection of workers in the establishments fixed with the labor regulation (delivers 2, title 3) which implement electric currents. This definition rises from the European legislation in this field.
The very low tension is the class of the electric tensions which cannot produce in the human body of dangerous electric current for the man.
- In alternative course: U ≤ 50 volts
- In D.C. current: U ≤ 120 volts
Characteristics
By experiment the human body crossed by an electric current of 30 my throughout one 30 second, suffers from ventilatory paralysis, and is considered ventricular threshold of fibrillation, main cause of died by electric shock.
The Very Low Working Stress (T.B.T.S.)
The security sources can be:
-
a transformer of safety conforms to standard NF INTO 60.742 or NF C 52.742;
- a engine-generator group;
- of the accumulators (piles) independent.
The use of these sources will depend on the buildings, the sites and the material used, the tensions maximum to be implemented are:
-
in the dry buildings: U alternate = 50 V and U continuous = 120 V
-
in the wet buildings: U alternate = 25 V and U continuous = 60 V
The secondary of the transformer (with dimensions use) should not in no case to be connected to the ground.
The masses of the electrical materials will have to be isolated from all the other masses, and not to be connected to the ground, nor with a driver of protection (P.E).
The Very Low tension of Protection (T.B.T.P.)
The design of installations known as T.B.T.P is identical to that of T.B.T.S. but the connection between the active parts and the ground with dimensions use exists.
The tensions maximum to be implemented are according to the sites:
- in the dry buildings: U alternate = 25 V and U continuous = 60 V
- in the wet buildings: U alternate = 12 V and U continuous = 30 V
The Very Functional Low tension (T.B.T.F.)
Is considered Très Functional Low tension any installation not corresponding, neither in the conditions of installations TBTS, nor in the conditions of installations TBTP.
Example: Food not having principal separation with active parts of another circuit, like the auto-transformer.
Other big factors
The separation of the circuits
The separation of the circuits applies in field B.T.A. and is established starting from a transformer of separation in conformity with standard NF INTO 60.742 or NF C 52.742.
The transformer has rollings up separated by a double insulation or a reinforced insulation; the separate circuit (with dimensions use) must have a level of raised insulation, no point of the circuit should not be connected to the ground. It is the same for the masses.
Exiguous enclosure
Definition: Room or site whose walls consist of metal or conducting parts, inside whose a person, because of exiguity of the place, has an important part of her body in contact with the conducting elements (space underfloor, interior of a silo, a tank, a boiler).
Condition of uses of a material
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portable lamps: they must be fed obligatorily in T.B.T.S. or T.B.T.P. ;
- tools electro transportable with hand : can be fed either in T.B.T.S. or T.B.T.P. ;
The material will be preferably of class II, in the absence of class I, if the mass of the apparatus is connected to the whole of the conducting elements of the enclosure.
In all the cases, the mains transformers must be placed outside the enclosures.
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Trouble lights of fortune: are completely prohibited. Only the use of material standardized NF C 71.000 and NF C 71.008 is authorized, whether it is fed in B.T., T.B.T.S. or T.B.T.P.
The index of protection must be at least I.P. 45, moreover the cable electric and the card of current must be nondismountable in order to guarantee a minimum I.P. Before use of a portable lamp, it is essential to check its good state.
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