Certain conductors are subjected to high voltage
a
M
gradients (approx. 1 kV/µs), which generate a
major current spikes through the stray
N
capacitance of circuits.
Leakage currents of a few tens or hundreds of mA can flow (common mode) and be detected by the RCD, as shown in figure 27 for a variable-speed drive.
Fig. 27 : RCD disturbance caused by high-frequency Unlike the 50 Hz - 60 Hz leakage currents for leakage currents.
which the algebraic sum is zero, these HF
Cahier Technique Schneider Electric no. 114 / p.20
An analysis of these phenomena and solutions to satisfy the constraints are presented in detail in Cahier Technique publication no. 204, LV
protection devices and variable-speed drives.
See also Appendix 2, Types of converters and fault-current waveforms.
Uninterruptible Power Supplies (UPS)
In installation with backup sources such as UPSs, the protection system must take into account the different possible configurations. In Modular version
particular operation on AC power or on the batteries, bypass switches closed or not, etc.
In the example in figure 29, the installation (TT
system) includes a UPS. If AC power fails, it is necessary to earth the neutral downstream of the UPS (i.e. close contactor K) to ensure correct operation of the RCDs.
However, this earthing operation is not
indispensable to protect persons because: c the installation becomes an IT system and the first fault is not dangerous,
Switchboard version
c the probability of a second insulation fault occurring during the limited time of operation on Fig. 28 : RCDs with HF-current filtering (Vigirex battery power is very low.
RH99M and RH99P from Merlin Gerin.
Non-priority
loads
3L
3L
N
3L N
3L
Automatic
3L
3L N
N bypass switch
AC bypass circuit
3L N
Manual bypass switch
N
Relay to detect
(Maintenance)
Priority
voltage failure
K
loads
Fault supplied by AC power
Fault supplied by UPS battery
Fig. 29 : When loss of AC power is detected, contactor K closes to recreate the TT system downstream of the UPS.
Cahier Technique Schneider Electric no. 114 / p.21
5 Conclusion
At a time when electricity has come to play an c For the protection of persons against indirect increasingly dominant role in residential, contacts, an RCD is:
commercial and industrial applications, it is v compulsory for the TT system,
useful to review and quantify electrical hazards v necessary for the IT system if there are several and provide information on residual-current earth electrodes,
devices (RCD).
v recommended for very long circuits on TN and As for all devices, they have their strong and IT systems.
weak points. Not yet fully perfected, they nonetheless play an increasingly important role c RCDs also provide protection against:
in the protection of life and property. All v fires of electrical origin. They are the only industrialised countries make extensive use of effective means to limit fire hazards caused by RCDs, with a variety of system earthing
tracking currents, whatever the SEA,
arrangements, in both industry and housing.
v destruction of machines in the TN system.
The following are the most important points to be Modern RCDs continue to progress in terms of retained from installation standards and current reliability and immunity to interference
practice.
phenomena that are not insulation faults.
c For the protection of persons against direct The purpose of this document is to further contacts, an RCD is not only very useful, but knowledge of RCDs and thereby contribute to often an additional measure required by
the safety of life and property.
standards, whatever the SEA. It is the ultimate line of defence in the protection of human life.
Cahier Technique Schneider Electric no. 114 / p.22
Appendix 1: calculation of touch voltages This section indicates briefly how touch voltages For more information, see Cahier Technique due to insulation faults are calculated, depending publication no. 172, Earthing systems in LV.
on the SEA.
TN system A Id
U0
N
D
PE
C
B
Rd
Ud
0.8 U
U
0.8 U
Ud =
o if R
= R and Rd = 0
Id
o
o
=
⇒
2
PE
ph
R
+ Rd + R
R
+R
AB
CD
ph
PE
In a 230/400 V system, the touch voltage Ud is therefore 92 V. This voltage is greater than the conventional touch voltage limit UL and represents a danger, i.e. the circuit must open.
In general, given the level of the fault current Id, opening can be initiated by the overcurrent-detection devices.
When the resistance values Rph and RPE are high or unknown, RCD protection is required.
Fig. 30 : Touch voltage for insulation faults on a TN system.
Cahier Technique Schneider Electric no. 114 / p.23
TT system
Id
U0
N
PE
Ud
Rb
Ra
U
R
Id
o
≈
Ud
a
= Uo
R + R
a
b
R + R
a
b
In a 230/400 V system, the touch voltage is approximately 115 V (if Ra=Rb). This voltage is greater than the conventional touch voltage limit UL and represents a danger, i.e. the circuit must open.
If the earth resistance is approximately 10 Ω, the fault current is approximately 11 A.
In general, opening cannot be initiated by the overcurrent-detection devices.