Possible Myford 7 safety issue

Quote " Arguing that if a machine is earthed it may make the situation worse is very questionable."

Absolutely not saying that but making the point that surrounding oneself with earthed metal surfaces does not neccesarily improve safety and can actually increase risk. Kitchen sinks are now no longer earthed for that very reason. Small kitchens allow hands to reach sockets and the sinks at the same time. It takes 2 connections to cause acurrent to flow so if one of those connections (the sink) is a high resistance path then less current will flow.

Metal pipework is no longer earthed. Plastic non conducting sections of pipe are common in houses.

The general philosophy is that if a metal box has mains fed into it via a cable then the box should be earthed. If it was an old Wolf metal cased drill then that would apply to the case and I would suppose everone would be happy with that. Attatching the drill to a metal stand does not increase the hazard.

I do however totally agrre with your comment "The first rule is prevention. So at no time should anyone come into contact with a potentially lethal voltage. "

When I was in the Electronics and Vibration lab, every Monday morning we would all check our resistance with the

Avo 7.

Normally from one hand to the other it was about 140K Ohms.

Under the weather, with a cold would show as 120K.. Standing on a floor insulkated to withstand 5K Volts, I inadvertently put the fingers of one hand across the Live and Neutral of a 13 amp lug. (The top came off! ) and am still here to tell the tale. 240 volts across 140K ohms gives a current of 1.7 mA., so may be I was lucky, courtesy of the floor since, electrically, I was "floating", and it was AC so the voltage fell to Zero on every cycle..

But Remember, "Its the volts that jolts, its the mills that kills"

We never went behind the main test panel when it was live with components at 300 volts DC.

No doubt, many of us have suffered a shock from the Kettering type ignition system of an engine. Fortunately, like flyback EHT for the Cathode Ray Tube TVs (NOT the 1930s models with separate mains fed EHT power packs! ) unable to provide sufficient current to prove fatal.

With modern ignition systems, that might not be the case, where we may not live to warn others!

As Martin said, Best never to be exposed to high voltages.

For this reason, in UK, site electrical equipment runs on 110 volts, fed from a centre tapped transformer, so the voltage is actually only 55 about earth.

So a mini lathe with a low voltage brushless motor may be safer than an older machine with a mains voltage motor, let alone a full three phase one.

Howard

Edited By Howard Lewis on 03/02/2022 12:12:56

What makes a shock dangerous? Definitely true "volts that jolts, its the mills that kills" or "Volts that Jolts, Amps that stamps". But in addition to how much current flows, it also matters where it goes, and for how long.

How much current is said to be dangerous varies considerably depending on who you ask. I was taught at school 30mA through the heart was usually fatal, while this summary I found on the web is more gung-ho:

1 mA Perception level. Slight tingling sensation. Still dangerous under certain conditions.

5 mA Slight shock felt; not painful but disturbing. Average individual can let go. However, strong involuntary reactions to shocks in this range may lead to injuries.

6-30 mA Painful shock, muscular control is lost. This is called the freezing current or "let-go" range.

50-150 mA Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible.

1000-4300 mA Ventricular fibrillation (the rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely.

10,000 mA Cardiac arrest, severe burns and probable death.

Touching the Live and Neutral pins in a plug with one hand isn't particularly dangerous because the current is isolated in the hand. Touching Live with one hand while standing on a concrete floor is far worse because the current flows to ground across the heart. However, even a mild local shock is dangerous if the recipient falls off a ladder or into rotating machinery!

The length of time current flows matters too. When the UK adopted 240Vac 50Hz rather than 120V 60Hz, it was argued the extra-voltage and lower frequency are equally safe because it tends to throw the victim off, whereas people stay connected longer to 120V and more current flows for longer. The throwing off effect doesn't apply to 440V systems, a nasty shock is almost inevitable.

The lowest voltage fatality I know of was caused by a 32V system. A well-soaked young coal-miner was running to get out of a downpour, when he fell heavily against a corrugated iron sheet behind which was a 32V power bus used to provide 'safe' electricity to a colliery machine. Not insulated because it only 32V. Killed because:

• The miner was hot and sweaty, which lowers skin resistance well below 140k ohms
• His clothes and the ground were both soaked, reducing resistance to ground
• The contact area between human and power bus was increased by the corrugated iron sheet, decreasing the contact resistance enormously.
• Current flowed until the power was turned off manually.

As is often the case, the accident occurred when a chain of circumstances came together which is why H&S practitioners are keen to put multiple protections in place. If one protection works, the chain is broken, and there's no tragedy. Here:

• Don't run in the workplace. (The employee broke a rule.)
• Understand the need to do a proper risk assessment. Risks aren't always obvious. (The employer was found negligent due to not considering what could go wrong. The installation was described as 'slapdash'.)
• Make sure power-lines can't be touched accidentally. (Basic requirement from the earliest days of electricity.)
• The accident occurred before RCDs were invented, but they cut the power automatically.

John raised the risk of a disconnected earth wire on his Myford. Actually the same fault can occur on any machine tool, not just Myfords. Vibration is really good at shaking wires loose and chafing insulation! I always check wires and connections when I'm inside or behind a machine.

Dave

Edited By SillyOldDuffer on 03/02/2022 15:06:29

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

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

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.

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

Correct!

Andrew