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

[abbey road d enfer]

I believe the situation is properly described in
http://en.wikipedia.org/wiki/Earthing_system

 

[jwhmca]

There isn't a lot of space in the thing. I'm not thinking I could fit a transformer in there... If I made a External ISO transformer and made the two go together, would I still need to ground the chassis, like in my second picture (RED Mark)?

 

[abbey road d enfer]

There isn't a lot of space in the thing. I'm not thinking I could fit a transformer in there... If I made a External ISO transformer and made the two go together, would I still need to ground the chassis, like in my second picture (RED Mark)?

If it is a really safe ISO xfmr (less than 120pF leakage capacitance), you could dispense with the ground, but I would not recommend it anyway.
The ground should be connected directly to the audio ground; the point you have indicated on your schemo is not always grounded (goes via bank of switches - I don't get it).

 

[PRR]

> situation is properly described in

That article is very broad.

And it is hard to interpret idealized line-drawings onto actual fuseboxes.

Further confusion: in the US each service (home) is required to have a dirt-rod or better. Apparently in the UK the earth is often "supplied by the company" (I assume a massive rod at the transformer).

For "Hot Chassis", all that is moot. If your home/studio isn't well grounded, one new toy isn't your big problem. This hot-chassis toy MUST have copper from chassis to Ground pin on wall-outlet. If you just do that, leave it hot-chassis, there's 50:50 chance it will blow the fuse when you plug it in. (0% if ALL wires and devices are done correctly, but I'd assume they are 60% wrong like my old kitchen.) So it is expedient to put an isolation in the Black/White (may be black-black {or blue/brown}) wires.

 

[Speedskater]

For a US type AC power system, then general rules are:

a] The only place the Neutral and Safety Ground (EGC/PE) are connected together is at the building Service Entrance /main circuit breaker box.
b] The Safety Ground must be securely connected to the chassis. No tack soldering.
c] The power switch must be on the Hot.  If both the Hot & Neutral are switched, then it must be an interlinked switch.
d] The fuse must be on the Hot.

correction

 

[emrr]

I have noticed old time radio guys on the cheap tend to suggest going to the salvage store and getting a used hairdryer with a GFCI plug, and salvaging the cable and GFCI plug for use on your radio/amp/etc.  Much cheaper than any other GFCI solution.  OF course an iso transformer is best if in budget. 

 

[JohnRoberts]

For a US type AC power system, then general rules are:

a] The only place the Hot and Safety Ground (EGC/PE) are connected together is at the building Service Entrance /main circuit breaker box.

Neutral and ground connected at SE/panel. 

b] The Safety Ground must be securely connected to the chassis. No tack soldering.
c] The power switch must be on the Hot.  If both the Hot & Neutral are switched, then it must be an interlinked switch.
d] The fuse must be on the Hot.

JR

 

[Speedskater]

For a US type AC power system, then general rules are:
a] The only place the Hot and Safety Ground (EGC/PE) are connected together is at the building Service Entrance /main circuit breaker box.

Neutral and ground connected at SE/panel. 
JR

Oops - that would have been exciting.
typo corrected.

 

[ricardo]

It's a looo.oong type since I looked at British Standards and they've changed all the numbers & names but ....

I think Neutral & Gnd are connected together at the Distribution Substation where zillion kV Delta distribution is converted to 230V Star.  Your house gets one of the phases and the GND/Neutral (the Star point).

I don't think you connect Neutral & Gnd at the Service Entrance (your house main circuit breaker box).

This means I ALWAYS expect a few VAC between Neutral & Gnd in a properly wired UK/Oz house.

I now know that if I ever have to work in da Rebel Colonies, if I find zilch between Neutral & Gnd, it is a sign of dem Rebels trying to kill off da Redcoats & loyal Colonials but only getting it right 50% of the time. 

 

[PRR]

>> Hot and Safety Ground .... connected together
> Oops - that would have been exciting.

I've done it.

In moderation.

New garage. New dirt-rod.

I wanted to know if it did any good.

I already know that bench ohm-meters can't read dirt-rods. Iron and copper in dirt generate a DC voltage higher than what DMMs use for ohms. Also stray return current over my land cause an even higher AC voltage, which a DMM's Ohms might ignore, but doesn't.

So I connected 124V HOT to Dirt-rod to see what current would flow.

Actually, to avoid great excitement, I put in a 250W lamp. And a 10r resistor. Worst could happen was a bright lamp.

What really happened was the GFI cut-out. Doh!!

Re-rigged with un-safe power. At first the lamp didn't light. Slowly grew to a dim glow.

Using two different lamps and figuring several different simplifications, I get 70-80 ohms per dirt-rod. Not bad for sitting on rock. Of course this is referenced to utility-rods, which are on the same rock.

So a dead-short from Hot to Dirt can flow, at most, about 1.5 Amps. More than enuff to kill, NOT enough to blow the smallest fuse in my box. A sustained short to dirt puts 150-200 Watts of heat in the dirt around an 8-foot rod, which isn't much.

I now actually have five dirt-rods in three groups (pole, garage, house) and probably ~~25 ohms to the general muck/rock around me.

I need to dig the well and bond that. I suspect it alone is near 25 Ohms, so in a lightning strike it will take a large part of the hit. If not bonded, the lightning stroke will bust-through the varnish on the pump motor; better to give it a sturdy all-metal path from overhead line to underground.

We are a long way off from a hot-chassis dictation machine, no? He could just stand on a wood floor and use NO other electric items at the same time. Or iso-former so he does not have a copper-path from chassis to Hot.

 

[JohnRoberts]

I think the more exciting consequence related to strapping ground to hot at the service entrance is that all the outlet grounds would be hot.  : :

JR

 

[zebra50]

I think Neutral & Gnd are connected together at the Distribution Substation where zillion kV Delta distribution is converted to 230V Star.  Your house gets one of the phases and the GND/Neutral (the Star point).

I don't think you connect Neutral & Gnd at the Service Entrance (your house main circuit breaker box).

This means I ALWAYS expect a few VAC between Neutral & Gnd in a properly wired UK/Oz house.

Hi Ricardo, that is how I understand it to be also.

Our studio / workshop is on a farm some distance from the substation, and we get between 3 and 8V between earth and neutral... which can be entertaining!

The individual circuit breaker switched at our main board only disconnect live, so if you cut a cable, the neutral voltage is sufficient to trip the main breaker.

So... turn off the ring main, cut the cable, out go the lights....Irritating!