No harm in providing all the safety that you did, but is it justified in ALL contexts ? CE at what cost ?

Dear Raghunathan,

I do not want to offend Russell, but there must be something wrong in his statement, when he mentions, that CE testing had damaged his transzorb.

CE compliance demands that circuits applicated in rough industrial environments must be robust at the terminals connected to 24VDC power supply line. So, immunity against 'bursts' and 'surges' is tested. This makes sense, because it is to be expected, that wide spread arround 24VDC lines can carry bursts and surges very well.

Immunity against surges is tested by applying well known pulses of up to 0.5kV via 42Ohm source resistance. Immunity against bursts is tested by applying well known burst pulse packages of up to 2kV via 50Ohm source resistance.
If you have a look at datasheet of transzorbs, then you will notice very fast, that withstanding of bursts and surges is absolutely no problem for a transzorb!

What Russell meant is: CE testing, especially according to 'low voltage directive' (I hope this expression is correctly translated...) demands, that plausible misconnections must not result in any relevant failure or damage. So, CE testers told Russell, that his circuit must be able to withstand an eroneous polarity reversal at 24VDC inputs. And maybe they were able to kill his transzorb because they delivered 24VDC with wrong polarity with idiotic current level. But this is not an indication, that a transzorb cannot do the job at this point as burst and surge suppressor, but that Russell's circuit was not properly designed to be able to withstand this situation. A simple fuse in series to this transzorb, or some other measure, would also have done the job.

The statement, that his transzorb failed and a MOV must be used to help out, leads to heavy misinterpretation, because a transzorb has a much tighter clamp-on voltage than a MOV. So, whenever parallel circuiting a transzorb and a MOV, the transzorb will always shunt out the MOV, means all the current will flow through the transzorb, because its clamp-on voltage is so much lower than that of MOV. You can remove the MOV and have the same performance!

I hope, I could convince you, that CE testing is not necessarily a mess...

Wish you a lovely weekend,
Kai