On 5/31/2015 1:14 AM, Phil Allison wrote:
bud-- wrote:
Phil Allison wrote:

** Connecting MOVs to ground like that in a *plug-in device* is prohibited on safety grounds under EU and similar regulations as followed in most 230/240 V places - like Australia where I am.

MOVs either have or can develop after some use leakage currents well in excess of the maximum permitted, which is only a milliamp or so for most categories of appliances.

The only components that may be so connected are agency approved " Y caps " with values like 4.7nF - while MOVs and " X caps" always go across the line.



Standard practice in the US for plug-in protectors is MOVs from H-N,
H-G, N-G.


** I really doubt using three MOVs like that is *normal* practice - certainly one never sees it in entertainment electronics or test gear, whether made in the USA or elsewhere.

Entertainment electronics may have some surge protection but is not
intended to protect against major surges.

As can be seen from the IEEE surge guide, 3 MOVs is absolutely standard
practice in the US.

Because of the additional risks added by having only H-N protection in
plug-in protectors (as in my last post) I suspect 3 MOVs is common
practice elsewhere, H-N only protectors can increase risk, not reduce
it. Gareth does not know what is connected H-G. You do not necessarily
know what the specific requirements are for plug-in protectors elsewhere.

All the packaging protectors come in that I have seen indicate what the
protection is (H-N, H-G, N-G). I suspect it would elsewhere. I am not
interested in looking.


The normal failure mode for MOVs is after sufficient energy hits they
start to conduct at lower voltages, eventually conduct at 'normal'
voltages, and go into thermal runaway.


** The fact that MOVs deteriorate means it is hazardous to have them wired from active to safety ground. Same argument goes for regular metallised film capacitors which also deteriorate when exposed to AC supply voltages - then explode.

In the US the likely maximum surge energy at a MOV in a plug-in
protector is a tiny 35J, and that includes for the largest probable
power line surge (as in my 1st post). The simplest UL listed protector
here will have a joule rating far above that.

The joule rating of a MOV is the single event energy that will put the
MOV at defined end of life (but still functional). Looking at MOV
ratings curves, if the MOV gets single hits that are far below that, the
cumulative rating is far above (like over 10x) the single event rating.
Failure is real unlikely. Coupled with high ratings, that is why some
companies can have protected equipment warranties.

The author of the NIST surge guide has written "in fact, the major
cause of [surge protector] failures is a temporary overvoltage, rather
than an unusually large surge." An example of temporary overvoltage
would be a high voltage distribution wire coming down on the 115/230V
secondary conductors.

You could also say it is hazardous to have leakage and fault currents on
the safety ground.