IMO Technical Brochure

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AC vs DC Safe Switching
As any electrician is aware the nature of DC switching has to be considered with care because on disconnection an arc can occur that
is more arduous than that produced with an AC load because there is no zero point on DC. The nature of this arc means that design
considerations have to be made within the switch in order to quench this phenomenon; that not only includes signicant contact gaps with
high speed of operation, but also thermal transmissive materials.
What must be considered is that any AC isolator is predominantly
designed with materials chosen such that the load will be AC. This means that the
load supply will be a 50/60Hz sine wave, whether it be 230VAC or 400VAC, etc. When
switching AC it should be remembered that the nature of the load supply will always
pass through ØVAC twice in every cycle and therefore although loads can be ardu-
ous in type the supply is self-extinguishing. By that we mean that even if the isolator
switches at peak load and an arc is formed between contacts, the action of the supply
reducing to ØV means that the load will tend to zero and the arc will be extinguished.
DC load, on the other hand, is always there and unless the load becomes zero, the
power being pulled through the contacts will always be the same. So if the load is 500VDC 25A it will be 500V 25A now, in 1s , in 1min, in
1hour – that is constant. In this case, unlike the AC above if you switch “OFF” on load you will also be switching “ON” on load; DC does not
go through a 0V level unless there is system supply failure (or some other fault).
So if switching a loaded DC circuit, especially at the high voltages that can be found in PV installations (up to 1000V or more), current will
continue to ow over the opening contact gap due to the partial breakdown of the air between the contacts. This phenomenon is viewed
as an arc between the contacts and it will only stop when the distance between the contacts, and so the air gap, becomes large enough to
prevent the continued electrical breakdown.
In order to replicate in DC, the self-extinguishing nature of AC, then switching OFF the load should occur quickly and in a switch that is
designed with a contact system that allows enough distance to break the DC arc and dissipate the arc energy present during such a
switching operation. Therefore, in order to perform such switching safely a fast operating switch-disconnector is necessary.
What is a Switch and what is a Switch-Disconnector?
We are all familiar with a switch. In its basic form we all know it as having one or more sets of electrical contacts that are connected to a
load and manually operated to either close or open the contacts in order to make them conducting or non-conducting.
However, there is a European standard covering switches and switch-disconnectors which is EN 60947-3, and in this document there are
denitions of industrial switches.
A switch is a mechanical switching device used for making and breaking current in an electrical circuit within
certain operational conditions.
A disconnector is a mechanical switching device used for carrying current in an electrical circuit under
normal conditions and for providing off-load isolation, therefore it is only intended to be used for isolation
once the current ow is negligible or has been interrupted by another device.
A switch-disconnector is a mechanical switching device that meets the requirements for utilisation as both a
switch and a disconnector, so it can be used to make and break current whilst also giving on-load isolation.
Electrical installations, whether it be residential or industrial, normally follow a set of regulations in order to
ensure a safe living or working environment. In the UK these rules are specied in the IET wiring regulations BS 7671. Within these regula-
tions Chapter 53 Section 537 covers the requirement for Isolation and Switching, whilst Section 712 contains specic requirements relating
to the installation of PV power supply systems including those with AC modules.
If a switch is not rated or classied as a disconnector or switch-disconnector then BS 7671 does not allow for its use in an electrical circuit
as safety isolation switch. EN 60947-3 is listed in BS 7671 Table 53.2 as an appropriate standard covering product isolation, emergency
switching and functional switching; and as IMO designs and manufactures its range of switch-disconnectors (more commonly referred to
as isolators) to this European Standard our range of Solar Isolators therefore meet the requirements stipulated under BS 7671.