LCD x-digit display insulation to user

[trobbins]

Some x-digit LCD display meters are finding their way in to metering applications where the LCD meter is sitting at some hazardous voltage with respect to the user.

I'm guessing all commercial handheld meters that use LCD displays would be using a supplementary insulation cover film of at least 0.4mm thickness, and with bonding around the display edges so that creepage/clearance is not a concern.  That supplementary insulation is probably a requirement for any CAT rating, even CAT I, and of course there are a myriad of test instrument related standards where that issue is likely defined within.

So my query relates to DIY use of such LCD displays/meters, and probably 'semi-commercial' or international commercial products that don't align themselves with specific test instrument standards, but are being used in applications that are essentially energy limited, secondary-side (ie. not mains side connection), but are galvanically connected to circuitry at above typical defined extra low voltage levels that have been used in various standards (like 42V, 50V, 60V, 120V, 150V).  I'm not referring to applications where the display is powered from a circuit near earth potential and probing a high voltage level, but rather where the measuring circuit/display is connecting to an equipment node that has a hazardous voltage with respect to earth.

It seems that the inherent glass cover of an LCD display is quite thin (eg. possibly below 0.4mm), but as long as it is not cracked, and there are no creepage/clearance concerns around its user accessible perimeter, then it provides a basic insulation function.

The query is should a supplementary insulation layer be added for any application where connection is above some defined low voltage level, and how does that relate to typically relevant standards for test/measurement equipment.

One application of interest is with valve/tube amplifier test gear, where DIY'ers are floating measurement devices up to 'output stage' anode voltage levels which can idle at a few hundred voltage DC (up to about 500-600V typically), but can also swing during operation to voltages well in excess of 1kV.