How Dangerous are Metal Bodied Powertools?

[Rhyolith]

I like to use old tools, mainly simply because they are more reliable and maintainable... each purchase with something made before ~ 1970 is for a lifetime rather than the next few years.

However when it comes to power tools I for a long time kept clear of anything secondhand (let alone stuff more than 10 years old). How things have changed.... :roll:

Metal Bodied Powertools by Rhyolith, on Flickr

I still get looks of horror when I say I use a belt sander from 1960 and several drills much older (1920s being the earliest i think).

However, the only powertool thats ever hurt me due to a fault (and consider i use the aluminium bodied tools a lot more than anything else now) was a plastic bodied B&D from the early 2000s... (it literally blew up!).

So this got me thinking, is the horrific reputation of metal bodied power tools actually earned?

The fact aluminium conducts electricity is probably the first thing that springs to mind when thinking about this and its one of the things that kept me off these things for a long time. However, if you think about it why would the electricity go through you? If there's an earth (which of course, there should be) then there is virtually no situation (to my knowledge) where a shock would go through you rather than the earth... Short of something really stupid like using the tool soaking wet or something #-o

Something else to consider is that, in my opinion most accidents caused by the tool itself (so not misses etc) are down to the tool failing physically, this was certainly the case with the aforementioned plastic drill. Its no secret that most of these older powertools and much better built than their modern counterparts (particularly makes like Wolf) so much less likely to fail.

Do you think metal bodied power tools are a death trap or is it just a theoretical risk that next to never actually happens? Maybe even the poor build quality of modern tools is actually more likely to harm you?

Thats a bit of a brain fart, but maybe it will interest someone.

 

[Rorschach]

If they are earthed (and checked and maintained properly) then you are much more likely to be injured by the tool "working" than failing :lol:

 

[Sideways]

No one ever sets out to make a mistake. Accidents do happen and materials fail over time.
I have seen many damaged power cords on portable tools and most people don't check the tool or the quality of the earth that it's plugged into often enough.
As old time metal tools do have an extra vulnerability compared to modern double insulated ones it makes sense to take the one simple, cheap and easy precaution - always run them through a 30mA RCD protected circuit / extension / even fit an RCD plug to the cord.
With that little precaution in place, enjoy some great gear
It tends to be heavy but motor speeds are often lower making it less offensive on the ears and the feel / balance of old kit can be nice.

 

[sunnybob]

The earth wire must be very thoroughly checked AND tested regularly with metal bodied machines. If they pass this test then youre good to go.
looking is not enough. I have seen many an earth wire that was visually perfect, but did not conduct a current.
Any cheap multitester will check earth continuity.
You set the tester to ohms on the lowest scale on the machine.
First, you need to test the tester because they are all different. Put the two probes together and check the reading. Make a note of that number.
Then you put one probe to the wall plug earth pin, and the other to a clean piece of the tools metal body (making sure its not resting on a metal table or your fingers are touching the body or probes).

Whatever that reading is, you subtract that from the reading of the two probes together and you want a number very close to 0.2 ohms.

Any number above 1 is dangerous to the operator.

But heres the REAL danger... if the machine develops a fault and the body becomes live because the earth wire is broken, and youre holding it with one hand the voltage will travel into your body and search for a way out. If youre barefoot on wet ground, it will zoom down and youll get a few burns but should survive.

However, as almost all heavy machines are two handed to operate, and most people wear rubber soled shoes, when the voltage goes into one hand the quickest way out is through the other hand and back to the machine.
Tell me, what organ is inside your body directly between your shoulders?

The heart will not survive mains voltage. Good bye.

While youre in test mode, use the same settings and if necessary a longer wire to reach, go around your shop and test from wall socket to wall socket by probing the screws that hold the socket to the box. Same numbers are required.
I have found sockets completely without earths in a room full of properly wired sockets.

 

[Eric The Viking]

As old time metal tools do have an extra vulnerability compared to modern double insulated

I'm afraid you've fallen for the sales doublespeak: "double insulated" actually offers a lot less protection than a proper earth to all the metal bits.

It was introduced to save manufacturers money, nothing more (at least one less bit of copper wire and less complexity in manufacture and testing).

The actual idea is ridiculous once you build things with exposed metal in proximity to live wires. The scariest tool I have presently is a cheap hot air gun, which is "double insulated" - a metal nozzle with live heater elements a few millimetres away (mica wire supports). When the last one failed it first spat out bits of mica. The same goes for soldering irons, kitchen lighting and so on.

I would very much like to see power drills with an earth to the chuck as a matter of course. As many of them are arranged, you can nick a live cable in a wall, make the chuck momentarily live, and if the wall is dry or hollow, there's no warning sign that the chuck is live.

Earths do two important things: they stop exposed metalwork reaching mains potential, AND they remove the live connection when a fault occurs (because the fuse blows or an RCD trips) . The idea of thick (enough) earth wiring in houses is so that fuses are forced to blow, and fast. I've added earths to some "double insulated" stuff simply because I've been worried about the electrical safety.

And even if the earth only goes as far as the cable inlet of the tool (i.e. you replace 2-core cable with three, but don't connect the earth), that adds an element of safety to the cable - damage is more likely to cause a fuse or RCD to trip, rather than expose live wires.

This I wholeheartedly agree with:

... it makes sense to take the one simple, cheap and easy precaution - always run them through a 30mA RCD protected circuit / extension / even fit an RCD plug to the cord.
With that little precaution in place, enjoy some great gear
It tends to be heavy but motor speeds are often lower making it less offensive on the ears and the feel / balance of old kit can be nice.

PS: Bob is absolutely right, incidentally about the danger of badly maintained and incorrectly installed electrics. I too have found missing earths and live-neutral reversals - public buildings, especially old or rewired ones can be really dodgy.

 

[Corneel]

No that last bit doesn’t work like that. With two hands on the metal body, they are both at the same potential. So no current will flow from one hand, through your body to the other hand and back into the metal body. The current will seek a path towards earth. So most dangerous is one hand on a live metal body and one hand somwhere on earth ( waterpipe for example). Your house needs an earth leakage disconnect thing to keep you save. Pretty normal here in Holland nowadays, but no idea what it’s called in the UK. It also saves you for leaky watercoolers or life, open wires etc.

 

[sunnybob]

I wont be volounteering to test your theory.

 

[Rorschach]

IF the body were to become live, it will only enter your body if you are earthed, otherwise it has nowhere to go.
Problem is of course, you won't know if it's live or if you are earthed until the last minute.

I recently worked (unknowingly) on a live lighting circuit. I was wearing electrically insulated boots and standing on a dry carpeted floor with nothing around me. So no electric shock as I was not a ground source. Not something to mess around with though.

 

[ED65]

Thanks for asking this Rhyolith! The replies have been hugely informative about something I know virtually nothing about. I now know how careful I have to be if I ever find one of those old B&D drills I've been after for ages.

 

[Eric The Viking]

Sorry Bob, you're outnumbered on the "two hands" thing:

It's live-to-earth that can be dangerous, BUT that is the reason for good earths on equipment. The idea is always to cause the fuse/RCD to blow as fast as possible.

In the case of hot wire fuses (i.e. pretty much everything that isn't a miniature circuit breaker, MCB), the blowing current is TWICE that of the rated current (few people know that - it should be taught in school science lessons!). The danger area is when there is a fault, but the earth resistance is too high to cause the fuse to blow fast (Bob mentioned this earlier - 0.2 Ohms being a good practical value). It gets hot but still passes a lot of current.

To be clear: 13A fuses actually blow at 26 Amps !!!

How do you die from electrocution? Hopefully you don't, but it's usually energy applied to the muscle and particularly nerves of the heart that do it, causing irregular contractions (fibrillation). Those paddle things apply a weaker shock, to get the heart back into normal rhythm again.

I have heard it expressed in Joules (Amps X time) or current (just Amps). Because it happens in a split second, the overall amount of energy needed isn't really relevant, as it's always massively exceeded. So for practical purposes, it comes down to the current flowing through the heart: I was taught that 20mA (milliamps or 1/1000th of an Amp) will do it (can't remember how much energy overall). The voltage helps achieve the fatal current, but volts do NOT kill you - current does.

RCDs for houses are set at 30mA - the assumption is that not all the flowing fault current goes through the heart, and that the trip can turn off the electricity before the fatal current is reached. A good earth means a stonking fault current, and a fast activation of the RCD (or the fuse blowing).

This is a good explanation, and it points out that it is not voltage that is the critical thing. Higher volts make it easier for the heart current to reach 20mA, but you can kill yourself with lower voltages, for example, a chain of only four car batteries gets you to 50V (when they are freshly charged), and there are umpteen examples of that voltage being fatal. So you could electrocute yourself clumsily trying to jump-start a lorry!

In that case Bob has a good, practical point: If the 50 volts was ACROSS right and left arms (through the hands), the heart is right in the middle AND you have useful conductors (arteries) completing the circuit! But if both hands were touching the same point (and the body touched nowhere else to complete the circuit), no damage would occur.

In theatres, film studios, etc. way back in the last century, the big dimmer boards for lighting were usually operated with a thick rubber mat for the operators to stand on. Without the earth reference it is much harder to electrocute yourself -- but not impossible (lorries).

The link above makes another point: electricity causes muscles to contract. Many people are electrocuted in circumstances they might otherwise survive because their fingers clench round the faulty thing, and they cannot make themselves let go - the flowing current overpowers the nerve signals. In incidents with onlookers, people trying to help have also been killed in the same way - they grab hold of the initial victim... and become victims themselves as they cannot let go.

This leads to literally three rules of thumb:

0. If you think something might be live, don't touch it until you are sure it isn't. That includes PEOPLE you think have been shocked. They might still be in the process of being electrocuted and, for a very modest investment in too-little thinking time, you can join them.

1. If you have to touch something you think might be live, use a knuckle, not a hand or a fingertip. If you get a shock, the muscle contraction should through your hand clear. Rule (0.) trumps this, every time though.

2. Use your right hand, as that side is slightly further from the heart (bad news for me as a leftie!), and make sure the other hand is NOT touching something likely to be earthed. Put it in your pocket, for example. But rule (0.) trumps this, too.

3. Electricians should not wear wedding rings (yes, fatalities have happened).

So I can't count? Well the first one is so blindingly obvious and important, it stands apart from the others.

I saw yet another American program* on a cheap-TV channel the night before last, when the presenters were gushing about an antique Wimshurst machine producing "really dangerous 40,000 power." Of course, that's why they used to be in every school physics lab, er not!

We are used to using electricity safely, but not teaching about it thoroughly, even today. It's understanding that keeps people safe, I'm certain, so they realise when something is dangerous, before they get to test it experimentally!

E.

*we are just as bad here, but I blame the producers for employing the ignorant as presenters. Wimshurst machines can actually be quite nasty, incidentally, which is one reason why Van De Graaf generators superseded them, but they meant 40,000 Volts, which on it's own will just make your hair stand on end. If current flows, that's a different matter...

 

[t8hants]

With the older drills, its sprained wrists that are the true worry, when the drill bit stops and the machine goes around then they are nasty especially if you have locked the trigger down and the machine winds the power cable up till it goes 'pop'.
I have seen one of the big Wolf machines throw two blokes across a deck when the bit jammed and the machine turned instead.
Change the cable if it is rubber insulated and insure you correctly connect the earth and all the joys of wrist breaking tools can be yours, but I'm going to stick to my battery drill.

 

[Corneel]

No that last bit doesn’t work like that. With two hands on the metal body, they are both at the same potential. So no current will flow from one hand, through your body to the other hand and back into the metal body. The current will seek a path towards earth. So most dangerous is one hand on a live metal body and one hand somwhere on earth ( waterpipe for example). Your house needs an earth leakage disconnect thing to keep you save. Pretty normal here in Holland nowadays, but no idea what it’s called in the UK. It also saves you for leaky watercoolers or life, open wires etc.

 

[AES]

Here anyway, and I guess it's the same in UK, plugs with a built-in ELCB (or RCD if you prefer) set at 30 mA are getting really cheap - about 20 quid or less equivalent. They come in 2 types, one you permanently wire on to the end of a cable, one which you just plug in just like an adaptor.

I've gone for these in a big way now, and as said in a previous post about quick release wiring for power tools, I've now got one RCD plug permanently wired on the four separate leads I've made up for this. But for "others" I've now got a couple of the plug in adaptor type.

You just need to remember to press the TEST button each time before use (on both types) and AFAIK, you'll be as safe as it's possible to be, whatever the tool.

Off Topic P.S: I've seen professional electricians apparently quite happily working on live wiring with the circuit still live. Being a devout coward that's NOT for me!

 

[Sideways]

As old time metal tools do have an extra vulnerability compared to modern double insulated

I'm afraid you've fallen for the sales doublespeak: "double insulated" actually offers a lot less protection than a proper earth to all the metal bits.

It was introduced to save manufacturers money, nothing more (at least one less bit of copper wire and less complexity in manufacture and testing).

The actual idea is ridiculous once you build things with exposed metal in proximity to live wires. The scariest tool I have presently is a cheap hot air gun, which is "double insulated" - a metal nozzle with live heater elements a few millimetres away (mica wire supports). When the last one failed it first spat out bits of mica. The same goes for soldering irons, kitchen lighting and so on.

I would very much like to see power drills with an earth to the chuck as a matter of course. As many of them are arranged, you can nick a live cable in a wall, make the chuck momentarily live, and if the wall is dry or hollow, there's no warning sign that the chuck is live.

Earths do two important things: they stop exposed metalwork reaching earth potential, AND they remove the live connection when a fault occurs (because the fuse blows or an RCD trips) . The idea of thick (enough) earth wiring in houses is so that fuses are forced to blow, and fast. I've added earths to some "double insulated" stuff simply because I've been worried about the electrical safety.

And even if the earth only goes as far as the cable inlet of the tool (i.e. you replace 2-core cable with three, but don't connect the earth), that adds an element of safety to the cable - damage is more likely to cause a fuse or RCD to trip, rather than expose live wires.

This I wholeheartedly agree with:

... it makes sense to take the one simple, cheap and easy precaution - always run them through a 30mA RCD protected circuit / extension / even fit an RCD plug to the cord.
With that little precaution in place, enjoy some great gear
It tends to be heavy but motor speeds are often lower making it less offensive on the ears and the feel / balance of old kit can be nice.

PS: Bob is absolutely right, incidentally about the danger of badly maintained and incorrectly installed electrics. I too have found missing earths and live-neutral reversals - public buildings, especially old or rewired ones can be really dodgy.

Hi Eric, I understand a degree of scepticism over the double insulated designs and I'm not impressed with the hot air example either but I feel you're making some overly broad assumptions and as in everything, there are manufacturers who do a better job and others who do a very poor job during their product design.

Let me reassure you that I've by no means "fallen for the sales doublespeak" but we're making superficial comments on an internet forum and the comparison was intended to get a message over....

Thanks for adding another voice to the RCD recommendation though, it is such a simple and cheap precaution to take there's no excuse not to.

Out of interest, do you have any electrical qualifications ?
"Earths do two important things: they stop exposed metalwork reaching earth potential " ?

 

[AES]

I don't know if it's relevant or not, but back in the 1970s, when (I think) double insulated tools were 1st introduced (certainly when I first became interested in DIY) I seem to recall reading one manufacturer's leaflet (a Bridges Drill I think - maybe?) saying that double insulated is overall SAFER than 3 core cable (earth) and on no account should one try to bypass the double insulation by running your own separate earth wire (or changing the cable to 3 core) because that was more dangerous.

I dunno, and only vaguely remember anyway, but something to think about? As per my last post, I'm sold on the built-in RCD plugs now.

 

[Eric The Viking]

"Earths do two important things: they stop exposed metalwork reaching earth potential " ?

That's a scary typo! I meant to write, "... stop exposed metalwork reaching MAINS potential."


Now corrected.

 

[Sideways]

This is a useful source for anyone interested. It is an educational thing produced by BEAMA a British trade association intended to explain what RCD's are and why they're a good thing. A few pages in it includes a more accurate explanation of our (people's) reactions to electricity. The effect of electricity on the human body depends on the voltage but also time of exposure.

https://www.google.com/url?sa=t&rct...BC5C58A94A2/&usg=AOvVaw3XA41VL-ybUixNEtkxES6l

It's a good idea to get into the habit of pressing the test button by the way although this is widely overlooked, you should press test and immediately reset the RCD's on your consumer unit every three months. It's a precautionary measure just in case of a problem with the mechanism.

 

[DTR]

In the case of hot wire fuses (i.e. pretty much everything that isn't a miniature circuit breaker, MCB), the blowing current is TWICE that of the rated current (few people know that - it should be taught in school science lessons!).
.......
To be clear: 13A fuses actually blow at 26 Amps !!!

So that would cover cartridge / glass-enclosed fuses too..... very interesting. That might explain some funny behaviour we've seen at work*. Thanks for sharing that. Can you give a reason why, or better yet point me to a source online please?

* electrical behaviour, that is. There's no accounting for my colleagues' behaviour.

 

[Eric The Viking]

I probably should have qualified it by saying "the hot wire fuses one normally comes across (including cartridge fuses, I think)":
https://en.wikipedia.org/wiki/Fuse_(electrical)#Characteristic_parameters

Note that the speed of blowing changes with the fuse type, but in each case it mentions twice the continuous current rating as the point the fuse blows. So presumably, what's written on the fuse is the continuous rating.

And they do get warm if running at the max continuous rating - it's an issue with UK plugs. In kitchens with big electric kettles or cooker rings plugged into a 13A socket, you often see scorching round the live receptacle, caused by the fuse heat at 13A (approx 3kW). The heat is conducted to the live pin, and thence to the plug and the socket faceplate. The neutral, which isn't physically in contact with the fuse, is usually fine.

I think the actual value for categorization is the I^2 t value (see the Wikipedia page), but the practical outworking for mains power circuits is 2x the rated current value.

 

[Sawdust Sam]

It is definitely not the case that a fuse needs twice it’s rated current to blow, it’s not as simple as that. With a normal fuse say in a plug or old consumer unit if current flows through it greater than its continuous rating then it will likely blow after a period of time although it true some fuses require 1.6 times rated current to blow. The key thing is how long it takes to blow with that time being inversely proportional to the current value. All fuses have a charateristic curve which shows how fast a fuse will blow for a given current.