Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Strange Grounding Problem with LV System
References:  <[email protected]>
Dale Farmer  writes:

> Martin Geraghty wrote:
> 
> > I work with an electrical installer who has turned to me for some idea on
> > how to solve a problem with a system that has a grounding problem that,
> > frankly, has me beat. If any of you guys can suggest something helpful, I
> > would be very grateful.
> >
> > The system is as follows. A lightprocessor 12 channel dimmer, that uses 3
> > channels for low voltage dimming (I think the other channels are used for
> > stage lighting). Of these 3 channels, channel 1 is used for 2 x 12v Stem
> > Lights from Basis
> > (http://www.basislighting.com/home/commerce/product/23/552), Channel 2 & 3
> > is for another 10 per channel of these lights. The electronic dimmable
> > transformer for each light is from Osram.
> >
> > The cabling for these LV lamps are installed within a metallic trunking that
> > is around the walls of the room at high level. At each position where one of
> > these Basis Stem lights are installed, there is a small metal box just below
> > the trunking that contains the transformer and connection blocks. The stem
> > light is fixed onto the lid of this metal box by means of a nylon insulated
> > connector that physically insulates the stem of the light from the lid of
> > the metal box.
> >
> > The trunking has been painted, but there are still unpainted grounding
> > screws available to get a ground reference from. All parts of the trunking
> > and the metal boxes have been electrically grounded back to one point back
> > at the dimmer position.
> >
> > The problem is this, if you touch the stem of the light from channel 2 or 3,
> > and touch one of the unpainted earth screws, you get a noticable electric
> > shock ( i measured it to be about 70Vac, assuming that the dimmer channels
> > are at 100%). If you do the same thing with channel 1, I still measure 70Vac
> > but i dont feel any tingle whatsover.
> >
> > We have tried bypassing the dimmer completely and connected 240V instead of
> > a dimmed output (the tingle was slightly less, measured the chassis to
> > ground vlotage at about 50-60Vac).
> >
> > The basis stem lights seem to be slightly strange in that one of the lamp
> > terminals seems to show an electrical continuity with the stem? Which means
> > that the stem of the light is directly connected to the LV output of the
> > transformer...Has anyone used Basis stem lights before?? is this normal??
> >
> > The system works fine, but you get a shock if you happen to touch the
> > trunking and stem at the same time...and the customer refuses to pay the
> > final payment until this situation gets resolved. Anyone got any ideas??
> >
> > Thanks for any help
> > Martin
> 
>     Looking at those light fixtures on the web site, they appear to be
> the kind that use the metallic body of the fixture as part of the
> current path.   

That's pretty normal on some low voltage lights. 

> Thus, these fixtures fall under a special hazard
> section of the electrical code. ( Check the code version you are
> working under for the details)  Usually these have to be mounted
> well out of the reach of everyone but properly qualified service
> personell.   Since the moving connections of this return path are
> not offering a zero resistance path back to the transformer, the
> metal of the lamps will carry nontrivial amounts of current, at
> whatever the supplied voltage is.

>     Not knowing the internals of the transformer, I cannot say
> how this is happening, but apparently the transformer is referencing
> the shell connection back to building ground, via a connection
> that has some electrical resistance.    When a person touches
> the building ground and the energized shell of the fixture, some
> of that lamp current will flow through that person, giving them
> the electrical shock you mention.


My quess for reason is different
In most low voltage halogen lamp transformers do not have 
ground referenced outputs. Their outputs are not earth referenced, 
they are floating transformer outputs (isolated from mains supply 
well enough). 

Thare is always some capacitance between incoming power live/neutral 
wires and the output of the transformer. This causes some 
capacitive leakage between mains input and the output side. 
Typicaly you can measure with high impedance multimeter (modern 
diigtal multimeters) from tenst of volts to over ground volts 
between ground and the incoming power. The capacitance is so 
small, that the current leakage at mains is limited to 
safe low value (a fraction of mA typically). You can feel 
this, but this small leakage current does not kill you. 
When frequencies get higher (for example dimmer output 
has high frequency harmonics) then the leakage current 
can get somewhat higher than with pure sinewave mains. 

I measured two 50W normal E-core halogen transformers and 
got around 50 pF on one (Midas) and 110 pF on another (Phonovox) 
as capacitance between the mains input and the transformer 
low voltage output. The leakage curren with this transformer 
from 50 Hz 230V mains voltage was around 5 microamperes 
(current between ground and output side when only multimeter 
between them), open circuit voltage measured with digital 
multimeter 45-50V (varied between different measurements). 

I also measured several electronic transformers. 
Movitrinic 9815A had around 125 pF capacitance between input 
and output. Leakage was around 55 microamperes and and voltage 
around 82V.
Ikea E0101 had around 105 pF and leakage of around 55 microamperes 
and the voltage around 90V. 
Aurora AU-K60 / Biltema 35-391 has around 1000 pF capacitance 
between mains input and low voltage output. I mesured around 
160 microamperes / 80 microamperes (depending on which one 
of output wires I connected I connected the multimeter to) leakage 
and around 206V voltage. Turning around the mains plug gave reading 
1.5V leakage voltage and 6 micropameres or 120 microamperes of
leakage current. 

>     Go back and *very carefully* read the manufacturer's
> installation instructions.  Does this installation follow all of
> the manufacturer's recommendations?   Go back and tighten
> all of your electrical connections, especially the ground and
> neutral ones.    Examine your lighting design with an eye
> towards safety.   Can it be reached and touched by anyone
> who may be considered not a qualified service person in
> the opinion of a greedy lawyer?   Remember how tall some
> basketball players are.
>     I'd seriously consider putting GFIs on these things.   I don't
> like lighting instruments that deliberately energize their structure
> rather than have to run an additional wire.
> 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/