Ground fault clamp thingummy. Sorry, non-political post.

[Keith Wootten]

Mah fellow Armenians,

Any advice on this test-bench safety problem?

I have some metal-cased equipment whose body should be nominally at
ground potential, held there by a cable shield. The cable is very long
and I can't ground the equipment body locally as the resulting loop
interferes with its operation.

On the test bench, it's possible that a fault (say in the cable shield)
could cause the equipment housing to reach an expletive-inducing high
voltage.

What I need is some sort of clip-on electrical clamp which grounds the
housing pending a reset if it gets more than a few volts away from the
local ground for a few milliseconds but otherwise leaves it more-or-less
floating by at least a few kilohms. Phew, that's a lot of fews. A
ground fault interrupter on the power supply won't be enough as there
may be considerable capacitance in the supply and cable.

It's not really a design query; I'm sure we could all come up with
something workable, and Fred's probably already posted six designs by
the time you read this, but this needs to be a commercial unit for
liability reasons. Is there something available to do this? What's it
called?

TVM

 

[John Woodgate]

I read in sci.electronics.design that Keith Wootten
'Ground fault clamp thingummy. Sorry, non-political post.', on Fri, 17
Sep 2004:

I have some metal-cased equipment whose body should be nominally at
ground potential, held there by a cable shield. The cable is very long
and I can't ground the equipment body locally as the resulting loop
interferes with its operation.

On the test bench, it's possible that a fault (say in the cable shield)
could cause the equipment housing to reach an expletive-inducing high
voltage.

What I need is some sort of clip-on electrical clamp which grounds the
housing pending a reset if it gets more than a few volts away from the
local ground for a few milliseconds but otherwise leaves it more-or-less
floating by at least a few kilohms. Phew, that's a lot of fews. A
ground fault interrupter on the power supply won't be enough as there
may be considerable capacitance in the supply and cable.

It's not really a design query; I'm sure we could all come up with
something workable, and Fred's probably already posted six designs by
the time you read this, but this needs to be a commercial unit for
liability reasons. Is there something available to do this? What's it
called?

Since you are in UK, I can tell you that it is almost certain that any
such expedient as you propose is illegal under Health and Safety at Work
legislation. It appears also to violate the earthing requirements of BS
7671, the IEE Regulations, which are legally binding in Scotland at
present, but will become so in England and Wales in the foreseeable
future.

Can you explain how this 'loop' arises, and what effect it has? How long
is 'very long'? This raises a 'remote earth' issue, possibly violating
'equipotential bonding' requirements. What sort of cable shield fault do
you have in mind?

There is almost certainly another solution to your problem which doesn't
fall foul of any safety requirements. But more information is needed in
order to pin it down.

 

[John Larkin]

Mah fellow Armenians,

Any advice on this test-bench safety problem?

I have some metal-cased equipment whose body should be nominally at
ground potential, held there by a cable shield. The cable is very long
and I can't ground the equipment body locally as the resulting loop
interferes with its operation.

On the test bench, it's possible that a fault (say in the cable shield)
could cause the equipment housing to reach an expletive-inducing high
voltage.

What I need is some sort of clip-on electrical clamp which grounds the
housing pending a reset if it gets more than a few volts away from the
local ground for a few milliseconds but otherwise leaves it more-or-less
floating by at least a few kilohms. Phew, that's a lot of fews. A
ground fault interrupter on the power supply won't be enough as there
may be considerable capacitance in the supply and cable.

It's not really a design query; I'm sure we could all come up with
something workable, and Fred's probably already posted six designs by
the time you read this, but this needs to be a commercial unit for
liability reasons. Is there something available to do this? What's it
called?

TVM

Git a big ole 35 or 50 amp bridge rectifier brick. Short the + and -
pins, and use the two AC pins as a bidirectional clamp diode.

If that's not enough of a drop, the next step is a big SCR or
something.

John

 

[Ken Smith]

Mah fellow Armenians,

Any advice on this test-bench safety problem?

I have some metal-cased equipment whose body should be nominally at
ground potential, held there by a cable shield. The cable is very long
and I can't ground the equipment body locally as the resulting loop
interferes with its operation.

When testing, could you unground the far end and ground the unit right at
the bench? This should remove the ground loop.

 

[John Woodgate]

I read in sci.electronics.design that Keith Wootten
'Ground fault clamp thingummy. Sorry, non-political post.', on Fri, 17
Sep 2004:

Mah fellow Armenians,

Any advice on this test-bench safety problem?

My response to your e-mail was rejected. The rejection message rubric is
copied below, with the e-mail address munged with (at) and exes.

This message was created automatically by mail delivery software (Exim).

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

keith.wootten(at)xxxntlworldxx.com
SMTP error from remote mailer after RCPT TO:<keith.wootten(at)xxntlw
orldxxx.com>:
host smtpin.ntlworld.com [62.253.162.40]: 550 Invalid recipient:
<keith.wootten(at)xxxntlworldxxxx.com>

 

[Keith Wootten]

When testing, could you unground the far end and ground the unit right at
the bench? This should remove the ground loop.

Yes, but you may be working on either end of the cable and still relying
on one ground. A clamp as described would be handy if one could be
found - the test engineer would simply clip it on to the unit under
repair knowing that it won't interfere with normal operation. It could
even incorporate some sort of warning if not connected, but as I said,
I'd like to find something commercially.

Cheers

 

[Ken Smith]

Yes, but you may be working on either end of the cable and still relying
on one ground.

Write into the test procedure that the ground must be connected on the end
that is under test. Make it clear that anyone not doing so will be fired.
If some gets electrocuted, fire them before they hit the ground

A clamp as described would be handy if one could be
found - the test engineer would simply clip it on to the unit under
repair knowing that it won't interfere with normal operation. It could
even incorporate some sort of warning if not connected, but as I said,
I'd like to find something commercially.

In the SanFansisco bay area is BART that had to solve this same sort of
problem with making the return current go back to the right substation.
Perhaps, you can try your local electric transit agency to see if they
have something.

 

[Mac]

Mah fellow Armenians,

Any advice on this test-bench safety problem?

I have some metal-cased equipment whose body should be nominally at
ground potential, held there by a cable shield. The cable is very long
and I can't ground the equipment body locally as the resulting loop
interferes with its operation.

On the test bench, it's possible that a fault (say in the cable shield)
could cause the equipment housing to reach an expletive-inducing high
voltage.

What I need is some sort of clip-on electrical clamp which grounds the
housing pending a reset if it gets more than a few volts away from the
local ground for a few milliseconds but otherwise leaves it more-or-less
floating by at least a few kilohms. Phew, that's a lot of fews. A
ground fault interrupter on the power supply won't be enough as there
may be considerable capacitance in the supply and cable.

It's not really a design query; I'm sure we could all come up with
something workable, and Fred's probably already posted six designs by
the time you read this, but this needs to be a commercial unit for
liability reasons. Is there something available to do this? What's it
called?

TVM

So, the equipment has a ground shield, but you want a local ground because
you don't trust the shield. This part really makes no sense to me. If the
equipment is designed correctly, why would the shield be faulty?

Can't you just test it before you start messing with the equipment?

You might object that testing the ground connection of the equipment is an
extra step. Since extra steps can be forgotten, the technician might be at
risk of shock if he or she forgets to test the ground. But connecting some
special local ground is ALSO an extra step which could be forgotten, so I
don't see how you win.

Besides, how is the local ground more trustworthy than the remote one? Is
it bare cable, bolted directly to a spike hammered into the Earth right
at the test bench?

Also also, unless the power cable is really ridiculously long, the
resulting ground loop shouldn't cause a problem. If it does, it probably
reflects a flaw in the equipment.

Sorry for being so negative, but the whole thing sounds a bit hokey to me.
If all of your fears are well founded, it seems as though the piece of
equipment you are talking about is very poorly designed.

--Mac

 

[Ken Smith]

On Fri, 17 Sep 2004 16:49:43 +0100, Keith Wootten wrote: [...]
So, the equipment has a ground shield, but you want a local ground because
you don't trust the shield. This part really makes no sense to me. If the
equipment is designed correctly, why would the shield be faulty?

I've seen the following:

(1) Because the shield has blown open due to an incorrectly applied mains
voltage.

(2) The thing did not get built according to the drawings.

(3) The ground of the outlet it is plugged into is not actually grounded.

(4) The other end of the cable is 4000 feet away and it really is
grounded but due to IR drops this end has +60V on it.

Besides, how is the local ground more trustworthy than the remote one? Is
it bare cable, bolted directly to a spike hammered into the Earth right
at the test bench?

Usually (where I work), all of the outlets have a "ground" pin and all of
the metal of the bench and the grounds of the equipment are all connected
to it. It really doesn't matter if it is ground or +500V so long as all
voltages are referenced to it. If someone brings a real ground connection
into the picture it really starts to matter.

 

[Rich Grise]

Write into the test procedure that the ground must be connected on the end
that is under test. Make it clear that anyone not doing so will be fired.
If some gets electrocuted, fire them before they hit the ground


In the SanFansisco bay area is BART that had to solve this same sort of
problem with making the return current go back to the right substation.
Perhaps, you can try your local electric transit agency to see if they
have something.

I've always wished someone had come up with a name of "Frisco Area Rapid
Transit."

 

[Jim Thompson]

[snip]

I've always wished someone had come up with a name of "Frisco Area Rapid
Transit."

They're trying to establish PHoenix Area Rapid Transit here (PHART),
but I think it will fail at the next election.

...Jim Thompson

 

[John Fields]

I've always wished someone had come up with a name of "Frisco Area Rapid
Transit."

 

[John Larkin]

[snip]

I've always wished someone had come up with a name of "Frisco Area Rapid
Transit."

They're trying to establish PHoenix Area Rapid Transit here (PHART),
but I think it will fail at the next election.

...Jim Thompson

Lake Tahoe has TART.

John

 

[Tony Williams]

Yes, but you may be working on either end of the cable and still
relying on one ground. A clamp as described would be handy if
one could be found - the test engineer would simply clip it on
to the unit under repair knowing that it won't interfere with
normal operation. It could even incorporate some sort of
warning if not connected, but as I said, I'd like to find
something commercially.

Didn't the old voltage-operated earth leakage trip
operate in exactly the way you require? It sensed
the voltage difference between true earth and the
'earth' wire in (say) domestic mains wiring, and
if the voltage difference rose above a few volts
it would trip. There was one in this house when
we first moved in, mfr'd by Chiltern afair.

 

[Tim Shoppa]

Any advice on this test-bench safety problem?

I have some metal-cased equipment whose body should be nominally at
ground potential, held there by a cable shield. The cable is very long
and I can't ground the equipment body locally as the resulting loop
interferes with its operation.

The standard solution (well, other than dyking off the ground pin
from the mains cable): two channels to scope, one of them the signal in
the tested equipment, the other the ground from the tested equipment,
and scope in differential mode.

Other than that, I can only recommend isolation transformers etc. as
a way of getting around existing ground loop problems, and even these
have safety issues if you don't properly do single-point grounding to
the clean side.

Tim.

 

[Mac]

On Fri, 17 Sep 2004 16:49:43 +0100, Keith Wootten wrote: [...]
So, the equipment has a ground shield, but you want a local ground because
you don't trust the shield. This part really makes no sense to me. If the
equipment is designed correctly, why would the shield be faulty?

I've seen the following:

(1) Because the shield has blown open due to an incorrectly applied mains
voltage.

Maybe I misunderstood the original poster. I guess I was thinking that
the piece of equipment under discussion was test equipment, and that is
why I couldn't understand the reason for the doubt. But maybe the
equipment under discussion is actually something that is in his shop
for repairs. In that case, it sort of makes sense that the ground
shield could be blown. Still, it seems as though it would be reasonable
to just check the ground shield with an ohm meter prior to plugging the
thing in.

(2) The thing did not get built according to the drawings.

Same comment as above.

(3) The ground of the outlet it is plugged into is not actually grounded.

Easily tested and fixed if it is your shop. Besides, you could say the
same of the local ground.

(4) The other end of the cable is 4000 feet away and it really is
grounded but due to IR drops this end has +60V on it.

It seems to me that the equipment under discussion should have a very
secure bonding of the case to ground by way of the three conductor power
cord. The ground conductor in the power cord should have a low enough
resistance that it will sink a LOT of current in a fault condition,
thereby causing a fuse or other protection to shut off the circuit and
remove the hazard. So the 60 V should disappear almost as fast as it
appeared.

Usually (where I work), all of the outlets have a "ground" pin and all of
the metal of the bench and the grounds of the equipment are all connected
to it. It really doesn't matter if it is ground or +500V so long as all
voltages are referenced to it. If someone brings a real ground connection
into the picture it really starts to matter.

Right. Where I work, almost all the equipment is grounded to the green
wire ground for the building. Some equipment uses two-wire plugs, but that
is relatively rare. Our benches aren't metal and aren't grounded except
weakly due to the static mats.

The building code requires the green wire ground to be connected to an
earth ground, though I've never actually checked it. (We don't use any
high-voltage DC equipment).

--

I don't know, the whole thing just sounded a bit weird to me.

--Mac

 

[Keith Wootten]

In message <[email protected]>, Tony Williams

Didn't the old voltage-operated earth leakage trip
operate in exactly the way you require? It sensed
the voltage difference between true earth and the
'earth' wire in (say) domestic mains wiring, and
if the voltage difference rose above a few volts
it would trip. There was one in this house when
we first moved in, mfr'd by Chiltern afair.

That's almost it, thanks, but I really want the tripped condition to
switch a ground to the unit under test; I presume the Chiltern device
removed power when tripped.

The cable is very long and even when operating normally, the IR drop
will ensure that the unit's body is not at local ground potential. There
is no possibility of using a different cable, and grounding the unit
locally stops it from working correctly. It's not a question of poor
design, rather of something working close to the limits of feasibility.
Sorry I'm short on detail, but you know the score.

Thanks for the help everybody, it pretty much confirms that there's not
an off-the-shelf solution to this. A bridge rectum-fryer, a zener and a
sensitive NC relay to detect the voltage and a second double pole relay
to self-latch and switch the ground connection through when de-energised
should do it, packaged in a wall-wart with test and reset buttons and
status lamps. Whether it's worth doing is a question for Ron.

Cheers