Switching of different voltages with adjacent contacts
Switching of different voltages by one relay is a commonly occurring application. An sample from practice
shows the risks behind the use of a relay type C9-A41 to switch 230 VAC and 24 VDC with the same unit.
Below you can see how to prevent problems.
An arc and a short circuit – the theory behind
The compact relay from the series C9-A41 is
equipped with four changeover contacts. In
our example, one contact was used to switch
a control voltage of 24 VDC with a maximum
current of 2 A, while the adjacent set of contacts had to switch a coil of a contactor with no
suppression module leading a current of 0.5 A
at 230 VAC. This circuit resulted in an arc and
a short circuit between the adjacent contacts.
The destruction of the relay and other surrounding components was the result.
The phenomenon which led to this incident
is based on the induction laws (Lenz›s law):
Commuting a coil also generates an inductive
voltage – directed against the applied voltage
- due to the change in magnetic flux. Depending on the phase at the moment of switching
off, this induced voltage may be in a range of
several kilovolts and creates a spark across the
relay contacts.
The result is the formation of plasma, the freed
carriers in the air are transmitting the arc on
the adjacent contacts and lead to a short circuit.
C5 – Series, RIC – Contactor.
Explanation based on the
standards
Solution
In relay technology in particular, the standards
EN 60947 and EN 61810 are used. The standard EN 60664 is referenced in the standard
EN 61810 and states that a basic insulation
with an air gap of 3 mm is required at a voltage
of 150 VAC. This corresponds to a pulse test
voltage of 4 kV. The air gap between adjacent
contacts in the relay C9 is 1.75 mm and the
test voltage is specified with 2 kV rms. This
explains why the use of this relay for switching
24 VDC and 230 VAC in the same unit is prohibited.
For this application, it is imperative to separate
the 24 VDC circuit and the 230 VAC circuit.
This can be realized by the use of multiple
relays or by a relay fulfilling the relevant standards. There the pulse test voltage must be ≥ 4
kV, which is reached by a bigger gab between
adjacent contacts. One possibility is the use
of relays from the C5 series. These relays are
pluggable and available with different features.
A good choice are the contactors from the RIC
series. They can be fitted with an optional expansion module. These modules include two
twin-contacts, a good option to switch small
signals in 24 VDC circuits.
COMAT AG | www.comat.ch | 3076 Worb | T 031 838 55 77 | Comat Productmanagement | 25.04.2013