Martin Kyte | 03/02/2022 10:37:19 |
![]() 3445 forum posts 62 photos | 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. "
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Howard Lewis | 03/02/2022 12:12:28 |
7227 forum posts 21 photos | 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 |
SillyOldDuffer | 03/02/2022 15:05:14 |
10668 forum posts 2415 photos | Posted by Howard Lewis on 03/02/2022 12:12:28:
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" ...As Martin said, Best never to be exposed to high voltages. ...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. 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:
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:
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 |
Andrew Johnston | 03/02/2022 15:36:02 |
![]() 7061 forum posts 719 photos | 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 |
Martin Kyte | 03/02/2022 19:02:01 |
![]() 3445 forum posts 62 photos | An extreme example but I'm sure these guys would not appreciate being anywhere near anything that was earthed. |
Robert Atkinson 2 | 03/02/2022 21:11:28 |
![]() 1891 forum posts 37 photos | 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". Robert G8RPI. |
SillyOldDuffer | 04/02/2022 16:10:14 |
10668 forum posts 2415 photos | 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
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Andrew Johnston | 04/02/2022 16:20:46 |
![]() 7061 forum posts 719 photos | 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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