Making "HOT CHASSIS" Safe. Help **New Schematic added**

[jwhmca]

Hi Guys,

I have this Geloso Tape machine that has a hot chassis. I would like to make this a safe as possible... without using a "Bandsaw"

some previous thread stuff... when I was repairing it. http://www.groupdiy.com/index.php?topic=51219.msg651111#msg651111

I understand I could just put in a 3-prong power cord... but I'm wondering if there is a way to install an ISO transformer? Or rewire the existing one to make it perfectly safe?

 

[JohnRoberts]

The simplest way is to add a 1:1 isolation transformer in front of their auto-former.

Adding a 3 wire line cord will not help much if the outlet is lacking a ground connection or miswired (I found several like that in my house). It might be cheaper to cobble a GFCI outlet inside the unit in series with the two wire line cord. That way it will trip if more than 5 mA or so of any mains current tries to leak to the environment.

JR

 

[jwhmca]

John,

It seems like this would work and would blow the breaker if it was plugged in backwards?

Green would be ground prong
White would be Neutral
Black would be hot and now switched...

If someone had a missed wired 3-prong outlet let it would put the hot to ground?

It still would be bad if they used a ground lift and it was flipped...

 

[bruce0]

This thread is making my skin crawl.  Please be careful, thoughtful, and redesign so that nobody gets killed. No offense intended.

 

[jwhmca]

This thread is making my skin crawl.  Please be careful, thoughtful, and redesign so that nobody gets killed. No offense intended.

Yep, that's precisely what I'm doing!!!

Seems like maybe the ISO idea is best? Couldn't you still get "shocked" even with an ISO?

 

[JohnRoberts]

Connecting neutral to ground is called bootleg ground and can be dangerous is several situations. Reverse bootleg where hot is tied to ground in the outlet can and does kill people.

To repeat myself I have outlets in my house with no ground connected, and hot-neutral reversed (not bootlegs but not wonderful). 

=======
The iso transformer that completely floats primary from secondary allows you to hard ground the circuit ground and chassis on the secondary side, with no risk of primary voltage getting through.

JR

 

[ricardo]

I think in Oz, what you call 'bootleg ground' is illegal and the electrician who did it would be prosecuted.

I can't think of any reason why anyone would want to do it.  Was it a legacy of some idiotic regulation?

It's also illegal to do your own electrical work in Oz unlike the UK.  Maybe that's a good thing.

 

[jwhmca]

Okay guys... How about like this? Is the earth ground in red right now?

 

[jwhmca]

bump

 

[JohnRoberts]

I think in Oz, what you call 'bootleg ground' is illegal and the electrician who did it would be prosecuted.

It is not to code and they should be.

I can't think of any reason why anyone would want to do it.  Was it a legacy of some idiotic regulation?

No... it is never OK, but some DIY electricians when installing 3 wire outlets into 2 wire homes, discovered that they can make the cheap outlet tests say it's OK... The cheap outlet testers should be illegal too.

It's also illegal to do your own electrical work in Oz unlike the UK.  Maybe that's a good thing.

I don't like overly restrictive laws, but incompetent DIY electricians can and do hurt people. Natural selection still at work, but they don't always kill themselves, so imperfect.

Could we outlaw stupid?  8)

JR

 

[JohnRoberts]

bump

Looks good to me...

Of course measure twice, cut once...

Measure for open circuit between ground and hot, and ground and neutral in line cord, before plugging in.

JR

 

[ricardo]

No... it is never OK, but some DIY electricians when installing 3 wire outlets into 2 wire homes, discovered that they can make the cheap outlet tests say it's OK... The cheap outlet testers should be illegal too.

.. and their manufacturers prosecuted.  How about the electric chair?

Could we outlaw stupid?  8)

We could start with Congress.  How about the electric chair?

 

[zebra50]

It's also illegal to do your own electrical work in Oz unlike the UK.  Maybe that's a good thing.

It is not quite legal here either now. You at least need to have it tested by a qualified sparky. Although of course some people don't!

 

[JohnRoberts]

No... it is never OK, but some DIY electricians when installing 3 wire outlets into 2 wire homes, discovered that they can make the cheap outlet tests say it's OK... The cheap outlet testers should be illegal too.

.. and their manufacturers prosecuted.  How about the electric chair?

I have actually been using this as a design problem for late night beer soaked circuit design sessions and it isn't trivial to detect bootleg grounds and even reverse bootleg with simple circuitry because the mains power is AC so measuring between hot and neutral does not reflect a polarity. The non-contact voltage sensors actually grab a external ground reference from the environment.

Could we outlaw stupid?  8)

We could start with Congress.  How about the electric chair?

I have long hoped that politicians could possess at least basic math and economics education. I am not holding my breath.

Their chamber was wired up with a media-matrix computer controlled sound system long ago, so some modification could probably selectively send low voltage shocks to the individual politician that makes the dumbest comment. As angry as I am they deserve to live, just not in Washington, DC, perhaps working in the fast food industry in their home state.

JR

PS I mean no offense to fast food workers.

 

[lassoharp]

I'd make sure the house or building has the safety ground connected in the outlets.  Install the 3 prong. 

If you decide to go with the ISO make sure you check what the situation is with any other gear it will be plugged into - like DAW interface or compressor.  Regardless of method chosen, if you still have a voltage difference between the two chassis it will pass through you if you happen to touch them both at the same time.

I personally prefer having everything 3 prong and plugged into an outlet with a working safety ground circuit.  That way you know all chassis share a common. 

 

[PRR]

> it isn't trivial to detect bootleg grounds and even reverse bootleg with simple circuitry

Interestingly, many receptacle GFIs do kinda detect N-G shorts.

I first noticed this working around live GFIs, tracing a random-trip in previous owner's work. Later on a partly-wired GFI. Finally found some vague text online.

Apparently they use a high-frequency oscillator to probe the N-G screws. If the line is many feet long, it is a Not-Short; if less, it is a Short (and the GFI trips-out).

"Simple" circuitry is not, of course, the issue for the mass-market. Modern GFIs can have dozens of opamp-like stages. All the designer's hair under the chair. But once prototyped, all that stuff fits in the one IC which a low-price GFI requires anyway, for trivial added cost.

____________________________

> electrical work in Oz unlike the UK

In the UK, there is an interesting jumper. The 'Neutral' *can* be un-strapped from the Earth. This is required at testing (initial or major mods) to verify connectivity. In US boxes this can be easy or nearly impossible. (Not to mention the two types of service panels: bonded and unbonded; with N+G bus or separate N and G buses.)

 

[abbey road d enfer]

No... it is never OK, but some DIY electricians when installing 3 wire outlets into 2 wire homes, discovered that they can make the cheap outlet tests say it's OK... 

It seems some countries (Norway in particular) have tolerated this when they made 3-prong plugs mandatory. Criminal IMO.

 

[JohnRoberts]

> it isn't trivial to detect bootleg grounds and even reverse bootleg with simple circuitry

Interestingly, many receptacle GFIs do kinda detect N-G shorts.

I first noticed this working around live GFIs, tracing a random-trip in previous owner's work. Later on a partly-wired GFI. Finally found some vague text online.

Apparently they use a high-frequency oscillator to probe the N-G screws. If the line is many feet long, it is a Not-Short; if less, it is a Short (and the GFI trips-out).

"Simple" circuitry is not, of course, the issue for the mass-market. Modern GFIs can have dozens of opamp-like stages. All the designer's hair under the chair. But once prototyped, all that stuff fits in the one IC which a low-price GFI requires anyway, for trivial added cost.

____________________________

I am very curious about this (detecting bootleg grounds)... using a HF AC would be more sensitive to wire inductance than simple resistance. I have done some searching for this with no luck. Apparently there are professional test devices that ohm out between N and G but can get false positives when outlets are too close to the electrical panel where N-G is bonded or if house uses too good (low resistance ) wiring.

JR

 

[bruno2000]

Back to the original topic......I have an old, no longer needed 1:1 isolation transformer that we used on a hot chassis TV 40 years ago.  PM me for details.
Best,
Bruno2000

 

[ricardo]

In the UK, there is an interesting jumper. The 'Neutral' *can* be un-strapped from the Earth. This is required at testing (initial or major mods) to verify connectivity. In US boxes this can be easy or nearly impossible. (Not to mention the two types of service panels: bonded and unbonded; with N+G bus or separate N and G buses.)

I'm not sure but I seem to remember in the UK, N & G are supposed to be unstrapped.  Something to do with star or delta 3-phase distribution and how its handled at substations.

Anyone have up-to-date knowledge of the UK standards & regulations?
____________________

I'm certain if yus Rebel Colonials had a referendum, yus would vote the Electric Chair (full lethal dose) for 'stupid in Congress'.  I hope you get the chance.