Possible Myford 7 safety issue

[Martin KyteParticipant @martinkyte99762]

An extreme example but I'm sure these guys would not appreciate being anywhere near anything that was earthed.

[Robert Atkinson 2Participant @robertatkinson2]

For electrical safety only exposed metalwork (conductive items) which could become live due to a failure of "basic insulation" need to be connected to the protective conductor "earth".
So if the mains cable goes directly into an enclosed (apart from ventillation holes) motor only the motor needs to be earthed. This is because the current can't come through an earthed motor housing make the lathe (or whatever) live. Note the lathe may coincidentally be earthed via the motor mounting. If the mains goes via a switch mounted on a metal stand then the stand should be earthed. Unless the switch is enclosed in a earthed metal box or a insulated box. Note enclosed includes the back of the box and having proper cable entry and exit protection by glands or approved mains connectors.
An old style "Dewhust" reversing switch is a special case in point. Many only have a single screw securing the cover. They have an earth connection for the base but the cover is floating. A credible accident scenario is screw comes loose and the operator fiddles with cover and it touches a live part. To meet sensible standards the cover needs an earth lead.
Fitting a mains powered work light to the lathe or metl stand may require additional earthing, particuarly if the lamp was not intended for this aplication.
This is of course separate from any earth requirments to prevent interference.

Robert G8RPI.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 03/02/2022 15:36:02:

Posted by SillyOldDuffer on 03/02/2022 15:05:14:

…throwing off effect doesn't apply to 440V systems…

Why not? Phase to ground is no different to single phase 240V. Phase to phase will be a larger voltage, which could cause a larger current. Although the above states that 120V has a bigger current than 240V: people are negative resistance?

Andrew

I wasn't thinking of 3-phase, rather the voltage at which things get dangerous, which has been much debated over the years. Also DC vs AC and the various Hertz choices.

For economy, electrical distribution should be high voltage – more the better. Unfortunately, high-voltage electricity isn't safe. So what's the highest voltage that's acceptably low risk, where most people survive without serious injury most of the time?

Many administrations believe 100-120V to be the maximum allowed in an ordinary home, perhaps allowing 220V twin phase as a special case for cookers etc. More have settled on 220-250V systems, and in practice these don't seem any more dangerous than 110V systems. Might be because 230V systems are better specified with stricter rules about earthing and power in bathrooms etc, but the argument was made that 250V 50Hz shocks tend to throw people off, disconnecting them before anything nasty happens. In that context, I'm only saying shocks in the 400V region don't throw people off – muscles go rigid immediately and victims can't disconnect themselves.

What voltages are present in a 3-phase system confuses me, but in a UK 3-phase Y with neutral, I believe phase to neutral to be 240V and 415V between phases (root 3 * phaseToNeutralVoltage). Is that wrong?

Confusing because 3-phase Y with Neutral isn't the only way power can be delivered and transformers might be be involved. Also because although the UK is nominally 230V single-phase (400V across phases), the system actually runs at 240V, and because some equipments assume 220V single-phase and 380V 3-phase.

Dave

[Anonymous]

Posted by SillyOldDuffer on 04/02/2022 16:10:14:

…phase to neutral to be 240V and 415V between phases…

Correct!

Andrew

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