KYORITSU 4140 - Manual (Page 15)

̶ 13 ̶
AccordingtotheInternationalStandardIEC60364,forTTsystems
the characteristics of the protective device and the circuit resistance
shall fulfill the following requirements:
Ra x Ia ≤ 50V Where:
Ra
isthesumoftheresistancesinΩofthelocalearthsystemand
the protective conductor for the exposed conductive parts.
50 is the maximum safety touch voltage limit (it can be 25V in
particular cases like construction sites, agricultural premises, etc.).
Ia is the current causing the automatic disconnection of the
protective device within the maximum disconnecting times required
byIEC60364-41:
-200msforfinalcircuitsnotexceeding32A(at230/400VAC)
- 1000 ms for distribution circuits and circuits over 32A (at 230 /
400VAC)
The compliance with the above rules shall be verified by:
1) Measurement of the resistance Ra of the local earth system by
LooptesterorEarthtester.
2) Verification of the characteristics and/or the effectiveness of the
RCDassociatedprotectivedevice.
GenerallyinTTsystems,RCDsshallbeusedasprotectivedevice
and in this case, Ia is the rated residual operating current I△n. For
instanceinaTTsystemprotectedbyaRCDthemaxRavaluesare:
Rated residual operating current I△n 30 100 300 500 1000 (mA)
RA(with touch voltage of 50V)
1667 500 167 100 50 (Ω)
RA(with touch voltage of 25V)
833 250 83
50 25
(Ω)
Shown below is a practical example of verification of the protection by
RCDinaTTsystemaccordingtotheinternationalStandardIEC60364.
Fig7-2