Mains Isolation transformers help

[Andy & Kim]

Hi guys, maybe you can clear something up for me. I use a 230-230v isolation
transformer between the mains socket and some sound equipment as the supply
is on a different ring most of the time from the equipment on the stage.
(amps etc.) I don't suffer from earth loop and also feel it makes things
safer. A few people on a sound group have suggested this is illegal and
dangerous as it disconnects the PE. Could somebody explain the isolation
transformer thing and tell me if I am in deed causing safety problems.

Regards

Andy

 

[Dale Farmer]

Hi guys, maybe you can clear something up for me. I use a 230-230v isolation
transformer between the mains socket and some sound equipment as the supply
is on a different ring most of the time from the equipment on the stage.
(amps etc.) I don't suffer from earth loop and also feel it makes things
safer. A few people on a sound group have suggested this is illegal and
dangerous as it disconnects the PE. Could somebody explain the isolation
transformer thing and tell me if I am in deed causing safety problems.

Regards

Andy

I can't speak to the legality, as I'm not familiar with UK laws.
Safety I can speak on. You don't mention what you do the safety ground
connection in the electrical supply. The ground must be connected to
the building electrical ground system, preferably the same ground system
as the rest of the equipment.
If you have disconnected the safety ground connection, then you have
set yourself up to have a dandy electrocution hazard. The purpose of
the safety ground is to provide a lower resistance path to earth for
fault currents than the one provided by your sweaty body. This diverts
most of the electrical shock away from you, and hopefully allows enough
current to flow to allow protective breakers or fuses to open up,
stopping the electrical flow before they set your gear on fire.
Power isolation transformers are a good solution to a few rather
uncommon problems. But what it sounds like you are doing is breaking
the safety ground connection and adding an isolation transformer.
Rather dangerous, and probably a waste of time and resources. You would
have better results by putting those resources into fixing the pin one
problem (see AES paper #48, I think) that your gear has, and don't screw
around with the power supply side of things. Work on the signal lines
where the fault currents are measured in milliamps, and audio level
isolation transformers only weigh an ounce or so.

--Dale

 

[Andy & Kim]

I can't speak to the legality, as I'm not familiar with UK laws. Safety
I can speak on. You don't mention what you do the safety ground
connection in the electrical supply. The ground must be connected to the
building electrical ground system, preferably the same ground system as
the rest of the equipment.
If you have disconnected the safety ground connection, then you have
set yourself up to have a dandy electrocution hazard. The purpose of the
safety ground is to provide a lower resistance path to earth for fault
currents than the one provided by your sweaty body. This diverts most of
the electrical shock away from you, and hopefully allows enough current to
flow to allow protective breakers or fuses to open up, stopping the
electrical flow before they set your gear on fire.
Power isolation transformers are a good solution to a few rather
uncommon problems. But what it sounds like you are doing is breaking the
safety ground connection and adding an isolation transformer. Rather
dangerous, and probably a waste of time and resources. You would have
better results by putting those resources into fixing the pin one problem
(see AES paper #48, I think) that your gear has, and don't screw around
with the power supply side of things. Work on the signal lines where the
fault currents are measured in milliamps, and audio level isolation
transformers only weigh an ounce or so.

--Dale

Thanks for your thoughts Dale. The transformer I use is a off the shelf unit
and there is no disconnection of the earth in the plug top or any of the
outlets. I think I am asking is if this gives me better safer protection.
Rather like a building site isolation transformer.

Andy

 

[Dale Farmer]

Thanks for your thoughts Dale. The transformer I use is a off the shelf unit
and there is no disconnection of the earth in the plug top or any of the
outlets. I think I am asking is if this gives me better safer protection.
Rather like a building site isolation transformer.

Andy

Protection from what? The main thing an isolation transformer as I
understand your usage, would protect you from power factor problems in
your electrical supply, or if you are in a particulary grotty area for
electrical power. So if you are next to a welding factory or something
like that it is worthwhile.
Read the AES paper on the pin one problem. Learn about ground loops
and how to troubleshoot them. Better information can be found on the
rec.audio.pro news group.

--Dale

 

[TimPerry]

Thanks for your thoughts Dale. The transformer I use is a off the shelf unit
and there is no disconnection of the earth in the plug top or any of the
outlets. I think I am asking is if this gives me better safer protection.
Rather like a building site isolation transformer.

Andy

Andy, in days of old TVs and radios were often built with "hot"chassies. it
was intended that these chassied be connected to neutral but it was possible
to get the hot/neutral reversed. the chasseis were "safely" protected by an
insulated cabinet however an unwary serviceman could be killed hence the use
of isolation transformers in the shop.

in addition in modern equipment some switching power supplys omit the use of
an isolation transformer thus the use of an isolation transformer is
recomended when servicing.

the whole purpose for this is so that if the tech is somehow grounded to
earth he can safely touch either AC wire (but not both) and not be shocked.

in your case, you are using a (small) line transformer at FOH (front of
house or the mixer position) to help break a ground loop to the power amps
and thus reduce or eliminate an unwanted "buzz". i would not think that
there was anything illegal about this (your local laws may vary) as long ts
the transformer is not overloaded or modified. it not much different (IMO)
then plugging in a battery charger.

if personal safety is an issue (remember even big transformers can short)
just add a GFI (GFCI) breaker after the transformer.

personally i avoid the issue by habitually running power from the amp rack
to FOH. i have a quad box of 1:1 audio isolation transformers for
contingencies. the move away from unbalanced amplifier sends helps too.

 

[Andrew Gabriel]

In the US, PE stands for "professional engineer." You would not want to wire
one into the circuit. :=) You made me guess but my guess is your PE stands
for "power earth" which to us would be the "system ground." You certainly

"Protective earth", but same thing.
Also called CPC "Circuit Protective Conductor".