It's cheap, but
is it safe??
A couple of weeks ago we were
doing a turnkey project for a customer, and the project called for
two relay outputs to drive 24VDC loads. To keep the cost to the
customer down, we decided to buy a 3rd party 2-relay relay board. It
was much cheaper than one of our own 8-relay/8 input XIRO16
When the boards arrived we took a
look and got a bit of a queezy feeling of something not being quite
right. The board was advertised as rated for switching
240VAC at up to 5A. However, the spacing between the printed traces
for the relay contacts and the traces on the drive (coil) side
looked far too close together for 240V. In the end we removed the
relays to get a full view. It turns out that the spacing between
coil side and contact side, hidden under the relay, is only
But does that matter?
If you are switching mains (120V
or 240V) on the contact side, and a user could touch any exposed
circuitry on the coil side, then YES it
matters. Someone could die, and if you are the supplier of the
end equipment you could be sued out of existence just for selling
it, let alone killing someone. Not to mention that you would never
get your product approved by any regulator authority.
There are legally enforcable rules
about the minimum trace-to-trace spacing that must exist between
mains circuits and Extra Low Voltage (ELV) circuits. In the US and
Australia it is generally 6.4mm (0.25 inches). In Europe it is 6mm
or 8mm, depending on the class of equipment. The controllers we made
that control day spa equipment on the Queen Mary II (water close to
electricity!) uses 8mm rules. On our regular XIRO16 relay board
the spacing exceeds 6mm.
Then there's the relay itself. The
relays in the board we bought in are rated at 1.5kVAC coil to
contact breakdown. The rules in Australia require 3.75kVAC, in
Europe sometimes more. The relays we use in the XIRO16 are rated
4kVAC coil to contact.
And our customer's project? Well,
in that instance we were only switching 24VDC at an amp or so,
so it was fine. The point, though, is that the boards we bought were
described by their maker in a way that implied they are suitable for
240VAC use, without any mention that doing so could create a
dangerous situation unless safety insulation is provided elsewhere.
It also explains why we sometimes despair at some of the low prices
charged by competitors ... they may be taking short cuts that
compromise safety or reliability.