What precautions do people here use when working with mains voltages?

[Mint.]

What precautions do people here use when working with mains voltages? Also any tips?

[vk6zgo]

How deeply involved do you intend to become with mains voltage?

There are some on this forum who are very concerned about measuring and/or testing in mains circuits,probably because that is what they do for a living.
Others look at the mains from time to time during their work,as that is usually the ultimate source of power for the stuff they are really interested in.

VK6ZGO

[Mint.]

Well I am planning on building a power supply. However I am slightly concerned about the transformer (240VAC to 24VAC).

[Lightages]

You won't be in minty condition if you start playing without some basic training. The safety of connecting to mains is not something you should read about. It should be demonstrated and a person who knows what they are doing should show you in person. There are so many little things to remember and teach that I would not pretend to be able to show you what is needed without forgetting a step or something important myself.

With that said, unless you can take a basic electrical course at a local college, it is better to just buy a power supply that outputs what you need.

Here are my tips for what it is worth if you insist on going it alone:

1. Keep one hand in your pocket. That way you will not cause a current to flow from one hand through your heart to the other should you touch something by mistake.

2. Wear long sleeve shirts while working so you don't accidentally touch skin to ground while working.

3. Assume everything is live and will kill you. Never assume that a circuit has no voltage present and check with a meter before doing anything. Capacitors can hold on to lethal voltages for a looooooong time.

4. Wear glasses or protective goggles.

5. Work with a friend if possible.

These are my tips for a beginner. Please take a course first if you can.

[don.r]

I use a DIY dim bulb tester mostly. I also have a 2.2:1 isolation transformer. The dim bulb acts as a current limiter, the isolation transformer as a voltage limiter. Together they are fairly effective. You can also use a variac in place of the isolation transformer. I never hook up mains until I am sure there are no shorts or other issues with the circuit. A little fear helps too.

Good point about caps. Large electros can pump out healthy amperages for a long time after power down. Discharge them first before working on them (I use an insulated screw driver to short the caps... there are better methods out there  ).

[alm]

Connecting a fairly large value resistor across electrolytics so they discharge quickly after power off is also a good idea. This doesn't mean that you shouldn't discharge the caps before considering them safe, but it does makes accidental discharges through body parts less likely. If it's a high voltage cap, I would recommend shorting them permanently, since caps can regain a significant fraction of their voltage after being discharged due to dielectric absorption.

[FenderBender]

It's tricky at first, but provided you have a wiring diagram from the transformer and know how to follow it, you're in business. Make sure you fuse it and have a switch on it, just in case. Sometimes I'll power a project up without either, but that's only when I'm really certain I've got it right and I'm too lazy to hook up a switch/fuse. But do as I say, not as I do.

[Uncle Vernon]

What precautions do people here use when working with mains voltages? Also any tips?

What precautions I use and what precautions I would recommend are entirely different!

Well I am planning on building a power supply. However I am slightly concerned about the transformer (240VAC to 24VAC).

To do this you should not be working with mains at all.

Ok tips:

1. Don't work on live equipment period! Your question shouts you do not yet have the understanding to mess with current that will kill you first and ask questions afterwards.

2. Have EVERYTHING you intend on plugging into the mains checked by an electrician and or a hands on electrical engineer.!

3. Use an ELCB or RCD outlet for initial power-up of any of your creations!

4. GOOGLE it! We have Australian design rules for how things should be connected. They far exceed the standards practised within much of the important junk we buy. Follow these and you will ensure a high level of safety.

5. Use a Metal enclosure and ensure all exposed panels are securely earthed. (about now some parrot will bang in about double insulation. Ignore this, while at beginner level earthing everything is the safest approach.)

6. Clamp your mains cord (preferably twice) and ensure you leave the earth wire at least  2" (50mm) longer than other terminations.

7. Use a two pole power switch and one rated at least double your intended load preferably 10A or better.

8. Use a manually resettable breaker or safety no-touch fuse holder in your mains wiring.

9. Ensure all wiring used for mains is rated to sufficient current with minimum 600V insulation. Rate insulation for any ELV wiring run close to mains wiring to a similar level.

10, Maximise segregation between mains and ELV wiring and whenever possible use a physical barrier from mains wiring.

11. Heat shrink any exposed mains terminations and secure all wiring with clips and cable ties

12. Look away when you first power up and make sure you have quick access to a further point of isolation when if the smoke and flame comes out.

13. Ensure large capacitors within your project or DUT have had sufficient time to discharge.

14. Work in a cool dry place, ensure there are no liquids about to be spilled, no tools, screws or miscellaneous metal item to fall across contacts or terminals.

15. Remove watches jewellery and metallic tongue and penile studs as appropriate.

If you progress in your hobby there there will ultimately be situations where you will need to do some work on live equipment. You are not there yet. Don't do it! And do get someone sufficiently skilled to check your work.

I've seen lots of home brew projects built far safer than any retail appliance and I've seen more than a few that look like they were built on contract for Dr Kevorkian. What kind of project would you rather build?

Nothing about mains work is rocket science but second chances are not always possible. Be careful don't be put off. And don't electrocute yourself, burning flesh smells awful and other hobbyists don't need any more draconian legislation on what we can and can not do.

[EEVblog]

Well I am planning on building a power supply. However I am slightly concerned about the transformer (240VAC to 24VAC).

The best advice for a first power supply is to simply avoid mains wiring. Build a lab power supply based on an AC or DC plugpack.

And heed every one of Uncle Vernon's thorough list of tips.

Dave.

[bfritz]

When working with mains AC voltages, I always recommend using an isolation transformer.

The Isolation Transformer, allows its' output side to be disconnected from ground.  When using one you need to make sure other devices attached to the circuit are not grounded either.  The idea is that if you acidentally touch one node in the circuit, your body may get a momentary charge transfer (remember to AC your body looks like a leaky capacitor), but very quickly the node you touch would become the same potential as your body, reducing the severity of any electric shock.

An Isolation Transformer does not remove all risk.  So, use the same precautions you would without the isolation transformer.  I have seen isolation transformer probably save a couple lives where people inadvertantly did something dangerous with little thought, and only got a little tingle because of the isolation transformer doing its' job.

[vk6zgo]

I'm sorry,but I can't go along with all the paranoia about mains supplies.

In the days before the prevalence of solid state equipment,built in mains supplies were the only practical way to supply power to home made Electronic equipment.
Hundreds of thousands of "homebrew" radios & other devices were made over tens of years,using transformer/rectifier supplies,with a vanishingly small accident rate.
Anyone with enough common sense to not injure themselves doing any of a hundred other everyday activities
can safely create mains operated Electronic Equipment by following a few simple rules:-

(1) The only mains connections you need to make for a transformer/rectifier supply are in the primary circuit,& these connections require the most care.

(2) Apply Uncle Vernon's tips.

I think his tips (2)& (3) require a little closer examination

UV(2)---Nice if you just happen to have a tame electrician or "Ginger Beer" on tap!
In many cases,you don't,so you may have to make do with an advanced hobbyist,or if you have to do it yourself,be like Santa,make a list of the things you have to check in the primary circuit,& check it twice,or thrice,or as many times as you need.
If you still aren't happy,apply  UV(2).

UV(3)----Again,nice if you've got an RCD ,but if you are in an old house,you may not have one.

Fear not,however,as by paying a bit above the normal price for an extension cord,you can get one with a built in RCD. Obviously,RCDs hard wired into your building are better,as they protect many power outlets.
We didn't have RCDs in my day,but I never connected the active line to the metal chassis---never have for that matter.
Still, I would have definitely used a RCD if theyhad been available.

I reiterate,mains wiring in Electronic projects is not like Crocodile wrestling,or milking venom from Taipans,& the
application of a fair degree of commonsense makes it a simple & safe activity.
After all,you have to learn about it sometime!
This does not mean that messing about with fixed building wiring is by any means as simple or safe,& I  believe many of the posters are addressing this from that perspective.

VK6ZGO

[Psi]

Be aware of pets, especially cats, if they can get to your work area they may get electrocuted walking over or sitting on something that's live.

When leaving the room, even for a sec, never leave anything switched on if it has live parts.
People and animals can easy touch things when you're away.

I went to get some tools and when i came back my cat was asleep on top of my project.
It was switched off and didn't use mains voltages but it really proved a point well.

[Uncle Vernon]

I'm sorry,but I can't go along with all the paranoia about mains supplies.

Yeah a bit like Black snakes are generally timid and will always try to flee, just a bugger when you inadvertently corner one of the buggers. Same applies to mains terminals, a bit of respect and they are quite safe, add ignorance, stupidity or blind faith and they can bite you bad. Bad as in final, no more chances.

In the days before the prevalence of solid state equipment,built in mains supplies were the only practical way to supply power to home made Electronic equipment.

In the days I got my drivers licence, cars only had front seat belts, should we head back to that piece of memory lane too?

Hundreds of thousands of "homebrew" radios & other devices were made over tens of years,using transformer/rectifier supplies,with a vanishingly small accident rate.

Which in some cases was more good luck than good management. Are you really proposing a knot in a two core mains flex is still sufficient?

Anyone with enough common sense to not injure themselves doing any of a hundred other everyday activities
can safely create mains operated Electronic Equipment by following a few simple rules

I can only agree but some things you are best to get some help with. Sure guys have taught themselves to fly helicopters and all kinds of crazy shit, but you forget you and I have both done some form of trade and industry training. Even if it wasdone in the days of the dinosaur.

(1) The only mains connections you need to make for a transformer/rectifier supply are in the primary circuit,& these connections require the most care.

True, and the safe way of doing this is second nature to many of use. For new chums there is lots to consider. Many listed in my previous post.

(2) Apply Uncle Vernon's tips.

They are tips or rules I've learnt. Ponder most and you can see the disaster that preceded their general adoption.

I think his tips (2)& (3) require a little closer examination

Everything deserves close examination. It's the Internet people write all kinds of good and bad advice here.

UV(2)---Nice if you just happen to have a tame electrician or "Ginger Beer" on tap!
In many cases,you don't,so you may have to make do with an advanced hobbyist,or if you have to do it yourself,be like Santa,make a list of the things you have to check in the primary circuit,& check it twice,or thrice,or as many times as you need.
If you still aren't happy,apply  UV(2).

Sparkies are like about like assholes (and some behave that way), cannot be too hard to fine one, lots of them are cousins and uncles after all. I'll concede that advanced hobbyist is just as able, but they can be equally difficult to locate. The problem being advanced is subjective, anything from the guy at Jaycar who must know stuff because he has a beard through to seasoned hams. Some are not as advanced as they think they are, which is why I was cautious to suggest it.

UV(3)----Again,nice if you've got an RCD ,but if you are in an old house,you may not have one.

A plug in one can be had for around $20, an good double pole plug in one for closer to $80. A lot cheaper than, and a lot safer than, an isolation transformer suggested elsewhere.

Fear not,however,as by paying a bit above the normal price for an extension cord,you can get one with a built in RCD.

see above

We didn't have RCDs in my day,but I never connected the active line to the metal chassis---never have for that matter.
Still, I would have definitely used a RCD if theyhad been available.

More fond recollections of core balance relays and a world connected through rewireable porcelain fuseholders. Yeah nowadays I like to use an RCD by default. Beginners are likely in to be in rented homes, student slums etc. I'd recommend springing for an plug in RCD.

I reiterate,mains wiring in Electronic projects is not like Crocodile wrestling,or milking venom from Taipans,& the
application of a fair degree of commonsense makes it a simple & safe activity.

Not when you know what you are doing, Steve Irwin would have probably approached Mains with the same trepidation you or I would approach a croc.

After all,you have to learn about it sometime!
This does not mean that messing about with fixed building wiring is by any means as simple or safe,& I  believe many of the posters are addressing this from that perspective.

I hear what you are saying VK6ZGO, we don't need the fun police, or bullshit OH&S which is nothing to do with safety anyway. I don't wan to be the fun police and don't want to suggest nonsense like tradesmen only wiring.

Many more people perish from suicide or road trauma each year than by crocodile attack or electrocution, even so both still deserve a degree of respect.

Dave's suggestion of a plug pack supply is probably the best way for a beginner's first few projects,  particularly now modern switch mode ones can deliver cheap useful current.

[Uncle Vernon]

Be aware of pets, especially cats, if they can get to your work area they can easily get themselves killed walking over or sitting on something that's live.

Never leave anything switched on that has live parts when you leave the room.
People and animals can easy touch things when you're away.

I went to get some tools and when i came back my cat was asleep on top of my project.
It was switched off and didn't use mains voltages but it really proved a point well.

Good point, Ditto inquisitive children. For those with little ones a particular warning about those otherwise innocuous IEC cords we have everywhere. I am told (could be urban myth) one of the original design parameters of Aussie cord sockets was that they be larger than a child's mouth. I know of one instance of sheer panic when toddler under Dad's desk was about to suck on a live IEC socket.  Nothing fun police, just common sense.

[IanB]

Bear in mind that MintyCondition comes across as a youngster of about 10-12 years old. With that age group different conditions apply compared to an adult.

When I was a young kid I once used a metal handled screwdriver on a terminal block while it was energized and got a nice belt out of it. Now if you had stopped me and asked me what I was doing, I could have told you it was dangerous. But as a child, all sense of danger went over my head. I just reached for the nearest screwdriver and used it, mentally filtering out any risk--as of course I was immortal, as all (think they) are at that age. As an adult, I would not even have a metal handled screwdriver on the bench where I could absent-mindedly pick it up in the first place.

So gear the advice according to the asker. With apologies to MintyCondition, we have seen the previous kinds of questions that have been asked and the level of existing knowledge demonstrated.

[Psi]

Be aware of pets, especially cats, if they can get to your work area they can easily get themselves killed walking over or sitting on something that's live.

Never leave anything switched on that has live parts when you leave the room.
People and animals can easy touch things when you're away.

I went to get some tools and when i came back my cat was asleep on top of my project.
It was switched off and didn't use mains voltages but it really proved a point well.

Good point, Ditto inquisitive children. For those with little ones a particular warning about those otherwise innocuous IEC cords we have everywhere. I am told (could be urban myth) one of the original design parameters of Aussie cord sockets was that they be larger than a child's mouth. I know of one instance of sheer panic when toddler under Dad's desk was about to suck on a live IEC socket.  Nothing fun police, just common sense.

It's a real problem with cats, because they can sense the warmth and seek it out for a place to sit

[Mechatrommer]

my advice on mains. wear high impedance grounding system for your body, ie rubbered boots/shoes, chair/bench/matte if you can get one, if you're on a bench mains project. stay dry and keep thinking... "mains, mains, mains, switch it off before handling it". my latest shock is from teared cable i moved around while sweating, no shirt, bare footed on concrete. beside the normal shock at the "impact point" i can feel it along the side of my body to the right feet. the home ECB tripped just because of the current flowing through me, lucky me still alive.

[don.r]

Bear in mind that MintyCondition comes across as a youngster of about 10-12 years old. With that age group different conditions apply compared to an adult.

When I was a young kid I once used a metal handled screwdriver on a terminal block while it was energized and got a nice belt out of it. Now if you had stopped me and asked me what I was doing, I could have told you it was dangerous. But as a child, all sense of danger went over my head. I just reached for the nearest screwdriver and used it, mentally filtering out any risk--as of course I was immortal, as all (think they) are at that age. As an adult, I would not even have a metal handled screwdriver on the bench where I could absent-mindedly pick it up in the first place.

So gear the advice according to the asker. With apologies to MintyCondition, we have seen the previous kinds of questions that have been asked and the level of existing knowledge demonstrated.

I didn't get that impression at all. When I was in my early twenties I was unscrewing a terminal block on a piece of shop equipment (a lathe, I think) and got the shock of my life. I hadn't realized the the unit was hardwired into the 240AC three-phase wiring. I was simply ignorant of the dangers involved as many people are, regardless of age, who are starting out in the hobby or profession. I think the OP demonstrates exactly what is needed. Slight concern and respect for what is potentially a dangerous undertaking but perfectly doable with sufficient precautions. He couldn't do worse than some of the commercial Chinese "lab" power supplies.

[ee851]

(1)  Do not put a metal tool near a mains circuit until you have verified that AC power is not applied.

(2)  Do not disassemble part of a mains circuit (for example, a receptacle box) until you have verified that AC power is not applied.

Use a neon light that you know works because you just tested it to show AC power.    Since an AC circuit component has three leads (hot, return, ground), you must place the neon light's two leads in at least three different configurations--unless you specifically know that one particular lead is earth-grounded, then you just check the other two leads--to verify that no lead is AC-powered.

[ipman]

Really, all. If you build a mains powered power supply, you will get only one wire which is dangerous: the live wire.
You don't have to be a qulified electrician to be able to isolate those wires in the primary side with a properly rated electrical insulation.
I am no qualified electrician, and tough I know a lot about rules, before knowing them in detail, I got in touch with dangerous live wires just 2 times in my whole life. And this was based just on the knowledge aquired by myself, a little bit more than common sense.

Sure, I strongly recomend reading and following electrical codes, they are very good sources of information.

[ee851]

Oh yeah, the most important rules I almost forgot:

Don't work on any AC circuit when:

You are tired, OR
You need to use the toilet, OR
You haven't had a coffee yet, OR
There is a member of the opposite sex in the same room, OR
You have a feeling something is wrong or might be wrong, OR
You are electrically connected to something that is earth-grounded, OR
You are wearing wet clothing.

[Simon]

well really it is common sense. I have to admit some people are paranoid but then not knowing the person asking it is hard to be confident in just saying use common sense.

It is mostly common sense and I was making mains power fire simulators with a neon lamp/capacitor oscillator circuit for nativities models when I was 15 on bits of wood with the parts screwed into terminal blocks.

To a great extend common sense applies. remember that you are dealing with for your purposes with a limitless power source, a great prospect and a dangerous one. use RCD's and fuses.

I have to admit I used to look away when powering things up, it was instinctive common sense, being a bit dramatic does not go a miss.

The bottom line is treat it with extreme respect and double check anything ( i use a meter to check staus on all wires to ground even after killing a main switch when I work on home wiring)

Double think every move

[Mint.]

Bear in mind that MintyCondition comes across as a youngster of about 10-12 years old. With that age group different conditions apply compared to an adult.

When I was a young kid I once used a metal handled screwdriver on a terminal block while it was energized and got a nice belt out of it. Now if you had stopped me and asked me what I was doing, I could have told you it was dangerous. But as a child, all sense of danger went over my head. I just reached for the nearest screwdriver and used it, mentally filtering out any risk--as of course I was immortal, as all (think they) are at that age. As an adult, I would not even have a metal handled screwdriver on the bench where I could absent-mindedly pick it up in the first place.

So gear the advice according to the asker. With apologies to MintyCondition, we have seen the previous kinds of questions that have been asked and the level of existing knowledge demonstrated.

I think it's incorrect to judge people's age by their knowledge. Not everybody knows electronics and you only learn physics in the later years of school, kids nowadays have all their phones and computers without knowing how they even work.

[Mechatrommer]

I have to admit I used to look away when powering things up

i thought i'm the only one doing this. but only when i power up my "new untested" circuit.

[don.r]

I have to admit I used to look away when powering things up

i thought i'm the only one doing this. but only when i power up my "new untested" circuit.

I still do that sometimes. I always wince (with correct tongue angle, of course).