BU I LDI NG SOLUT IONS
— Protection against indirect contacts
Protection against indirect contact is one of the fundamental safety requirements of electrical systems.
The use of differential current protection devices for protection against indirect contacts is not compulsory in TN systems, but certainly represents an effective solution in the context under consideration. The use of residual current devices with a rated tripping differential current not exceeding 30 mA, is recognized by the CEI 64-8 standard as additional protection against direct contacts in the event of failure of other protective measures or negligence on the part of the users.
Protection against indirect contacts can be achieved in various ways (by automatic protection of the interruption, Class II electrical components or equivalent insulation, etc.), but in office buildings, the automatic interruption of the power supply is definitely the most commonly adopted solution. The differential circuit breaker plays a key role in the protection against indirect contacts by automatic power interruption with particular reference to the protection in TT and TN systems. In TT systems, in fact, differential current protection devices must be used for the automatic interruption of the power supply; the use of overcurrent protection devices for protection against indirect contacts in TT systems is in fact not permitted by standard IEC 60364.
In particular, additional protection against direct contacts is required:
• in rooms used for residential purposes for circuits supplying plug sockets with a rated current not exceeding 20 A, and • for circuits supplying plug sockets with a nominal current not exceeding 32 A intended for supplying mobile consumer appliances used outdoors.
In particular, the following condition must be met:
R E x I dn ≤ U L
where: • R E is the resistance of the earth electrode in ohms; • I dn is the rated differential current in amperes.
Earth leakage circuit breakers can also be used as a means of protection against the ignition of fire, being able to detect the degradation of the insulation of circuits and equipment, prodrome of a short circuit.
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