BECKHOFF AX2500 - Manual (Page 31)

6.9 Shock-hazard protection
6.9.1 Leakage current
Leakage current via the PE conductor results from the combination of equipment and
cable leakage currents. The leakage current frequency pattern comprises a number of
frequencies, whereby the residual-current circuit breakers definitively evaluate the 50Hz
current. For this reason, the leakage current cannot be measured using a conventional
multimeter.
As a rule of thumb, the following assumption can be made for leakage current on our
low-capacity cables at a mains voltage of 400 V, depending on the clock frequency of the
output stage:
I
leak
= n x 20mA + L x 1mA/m at 8kHz clock frequency at the output stage
I
leak
= n x 20mA + L x 2mA/m at a 16kHz clock frequency at the output stage
(where Ileak=leakage current, n=number of amplifiers, L=length of motor cable)
At other mains voltage ratings, the leakage current varies in proportion to the voltage.
Example: 2 x servo amplifiers + a 25m motor cable at a clock frequency of 8kHz:
2 x 20mA + 25m x 1mA/m = 65mA leakage current.
Since the leakage current to PE is more than 3.5 mA, in compliance with EN50178
the PE connection must either be doubled or a connecting cable with a
cross-section >10mm² must be used. Use the PE terminal X0/3 and the PE bolt in
order to fulfil this requirement.
The following measures can be used to minimise leakage currents.
— Reduce the length of the engine cable
— Use low-capacity cables (see p.41)
— Remove external EMC filters (radio-interference suppressors are integrated)
6.9.2 Residual-current circuit breakers (FI)
In conformity with DIN IEC 60364-4-41 – Regulations for installation and EN 60204 –
Electrical equipment of machinery, residual-current circuit-breakers (called FI below) can
be used provided the requisite regulations are complied with.
The AX2500 is a 3-phase system with a B6 bridge. Therefore, FIs which are sensitive
to all currents must be used in order to detect any D.C. fault current. Refer to chapter
6.9.1 for the rule of thumb for determining the leakage current.
Rated residual currents in the FI
10 -30 mA
Protection against "indirect contact" (personal fire protection) for stationary
and mobile equipment, as well as for "direct contact".
50 -300 mA
Protection against "indirect contact" (personal fire protection) for stationary
equipment
Recommendation: In order to protect against direct contact (with motor cables
shorter than 5 m) we recommend that each servo amplifier be protected
individually using a 30mA residual-current circuit-breaker which is sensitive to all
currents.
If you use a selective FI circuit-breaker, the more intelligent evaluation process will pre-
vent spurious tripping of the circuit-breakers.
AX2500 Product Manual 31
BECKHOFF
06/2007
Technical description