Safety tips for using mains power

[shadewind]

I'm working on a project to install some lighting in guitar amplifier head (that might sound weird because it will look neat and I don't want have an extra power plug just for the lighting. I thought about using the power supply from the amplifier but I don't want to mess with that and possibly (probably) introduce noise and interference to the amplifier output. So I was thinking I could add my own power supply to this and learn something in the process.

So my thinking was using a transformer from 230V to 12V AC, rectify, filter and then use a switching regulator to give me a stable 5V output. Maybe not the best solution, I don't know, but that's not the point really. I believe in learning by experimenting so we'll see.

But since I'll be experimenting with mains power, safety is very important and something I take very seriously. Do you have any tips on working with mains powered projects and power supplies? Safety precautions, tips, tricks?

[mikeselectricstuff]

Why don't you just buy an off-the-shelf PSU? Will be easier and cheaper than building something.

[shadewind]

Simply because then I will have learned nothing. But sure, I might end up doing that in the end.

[Psi]

- Use a RCD
- Keep one hand in your pocket when live 230v parts are exposed
- Its a good idea to have another person nearby who can see/hear you and give CPR if its necessary.
- Don't think of it just as a 'circuit' when it's live, think of it as a loaded gun pointing at you.

That being said, while 230VAC can definitely kill, most people on here will have had multiple shocks from mains voltage and are still here.


This next tip can be used to protect a circuit from faults while you design and test it.
- You can put an ordinary 40-200W mains voltage lightbulb in series with the 230v to your circuit, that way you form a current limit. Normally the current will still be enough to kill but it will stop the components from exploding instantly if you get something wrong, instead the lightbulb will illuminate. It will protect your components from damage and could save you from an eye injury. You need to pick the right wattage lightbulb so it doesnt illuminate with normal operating current.

This trick doesn't work so well for some switchmode power supplies, as they want to draw a huge current at the start and common wattage lightbulbs don't allow enough current for the switchmode to start up. A really high wattage lightbulb that would alllow enough current for the switchmode to start wouldnt protect it much when the current is much lower during normal operation.

[Alex]

Here is an idea, find a wall transformer that outputs 12V AC, then rectify, filter and regulate if needed. There are many such wall adaptors and this solution will allow you to learn without being exposed to mains voltage.

Here is what I mean, I am sure you can find them locally.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0458923

[Jimmy]

Op

If I get your Idea correctly you want to install this system into you amp box with only pulling 230v from the plug

If this is what you want to do open up your amp to see how much space you have behind the pannel to work with.

You can use an old PSU from a computer if you want it will be realy cheap. You wont need the fan for cooling if you are just going to run a couple of leds and a micro controller off of them. You would just cut an old psu power lead and wire it into the internal of the amp so you get power to the psu.

Then Brige Green and Ground to turn on the psu just like the computer would.

I would spend more time on the actual lighting than the supply you could use a Arduino to control some rgb led's that can do cool lighting effects

[shadewind]

Exactly, I want to draw 230V directly from the plug (preferrably, I want to include the power switch into my own circuit so that the lighting turns off with the power). A PC PSU is too big to fit in the case. Preferrably, I want to hide it behind one of the large transformers in the amp so that my own power supply is not visible.

The lighting I've got pretty much figured out. I have an ATMega8A-PU, a couple of Cree RGB LEDs which I'm gonna drive with PWM and some transistors.

[Lance]

- Use a RCD
- Keep one hand in your pocket when live 230v parts are exposed
- Its a good idea to have another person nearby who can see/hear you and give CPR if its necessary.
- Don't think of it just as a 'circuit' when it's live, think of it as a loaded gun pointing at you.

That being said, while 230VAC can definitely kill, most people on here will have had multiple shocks from mains voltage and are still here.


This next tip can be used to protect a circuit from faults while you design and test it.
- You can put an ordinary 40-200W mains voltage lightbulb in series with the 230v to your circuit, that way you form a current limit. Normally the current will still be enough to kill but it will stop the components from exploding instantly if you get something wrong, instead the lightbulb will illuminate. It will protect your components from damage and could save you from an eye injury. You need to pick the right wattage lightbulb so it doesnt illuminate with normal operating current.

This trick doesn't work so well for some switchmode power supplies, as they want to draw a huge current at the start and common wattage lightbulbs don't allow enough current for the switchmode to start up. A really high wattage lightbulb that would alllow enough current for the switchmode to start wouldnt protect it much when the current is much lower during normal operation.

Oh sure. I haven't been hit by 230 though. I got a shock from 120 V. My hand felt a little funny, but it wasn't that bad.

[BrickBoiler]

Probably the best tip I can give you is to never lose your attitude. Safety is important and so is learning, even if that means doing things the hard way.

The second best tip I know for line voltage is to use alligator clips. De-energize what you are working on, verify with a good non-contact-voltage tester, attach your alligator clips, and then power it up. Your hands never go near exposed live conductors.

[Zero999]

I did write a summary of basic tips for safely designing mains projects but didn't complete it and can't post it at the moment as it's on another computer. I'll post it some other time.

All I can say is follow the following basic guidelines:

Don't, whenever possible use a pre-made wall plug power supply.

If you have no choice then do the following, use some common sense and you should be fine:

• Use a power transformer with built in thermal protection or a fuse.
• Fit as lower current rating fuse as possible in series with the live conductor.
• Keep the mains as far away from the secondary side and user as possible, which means having at least 5mm spacing between 230VAC and extra low voltage on the PCB and 7mm for rectified mains (325VDC).¹
  
• Use a non-conducting plastic enclosure whenever possible.
• If you must use a metallic enclosure then connect it to protective earth/ground.
• Ensure there's at least 2.5mm of creapage between the mains conductors and the protective earth/ground conductor on PCBs or metalwork, 3.5mm for rectified mains.²
  
  
• Keep hot surfaces away from flammable materials and ensure the enclosure doesn't get hot enough to burn the user.

¹ Minimum requirements for double insulation.
²  Minimum requirements for basic insulation.

Disclaimer
If you don't understand the above then don't build a mains powered circuit.
This is only a basic guide, seek advice from the relevant bodies for getting your project manufactured.
And don't blame me if you kill/injure yourself or anyone else.

[shadewind]

I was first thinking that I could do this on a perfboard but I'm starting to realize that it's probably not a very good idea. I'm probably going to need a proper PCB which is going to be too expensive or too much work for this project (yes, really). I don't know if I could mount the transformer on something else maybe since it does have what seems to be mounting holes through the magnetic core. That's about the only thing that is really connected to the mains. Hmm... I have my doubts.

The thing is, it would be much less of a hassle if there was only one single power cable to the amplifier. On the other hand, it might not be such a good idea to mount a power supply directly behind another large transformer becuase of magnetic flux leakage and whatnot.

[Alex]

Don't, whenever possible use a pre-made wall plug power supply.

Do.

[Mechatrommer]

Don't, whenever possible use a pre-made wall plug power supply.

Do.

i also had similar answer as you Alex. but when some discussion arise on lightning strike, it made me think twice, as i usually made my own simple ac-dc adapter using transformer. even though well isolated, but the spike could be transfered to secondary side damaging anything without proper protection circuit. so next time i build my adapter, i will study safety and protection circuit more.

as for main safety. i know long time ago how electrocution felt like, but just few weeks back while installing my home lightings, i've been struck by mains several times, guess i never learn, even though knowing the safety, sometime i missed it. so thinking, maybe the best way to handle mains is to remember carefully everytime to switch off the main supply before handling it (the circuit).

[Jimmy]

use alligator clips

I would never even concider using alligator clips on one of my projects it is way too dangerous.


NO psu then thats the easy route gone. What you can do is buy a 240v to 24v transformer as they are usually common and cheap. On the primary side put a fuse and  a 300v varistor after fuse and to n of the primary side that will take care of lightning strikes then you can just use a small brige wave rectifier and a voltage regulator.

You can keep your component count low and just buy a pcb prototyping board "like a breadboard" and hack it together as you are only making one.

IF you decide to go ahead with it just rember if it looks dogey then there is potential for a large risk to your health. Buy a fuse holder dont make one use proper insulated wire. Put your swich on the secondary side of the transformer. Send us photo's if you are not sure on something

[Simon]

the mains can kill but being sensible will keep you safe. Do always have an RCD in operation, my lifes been saved by one before.

Yes the best way to think of the mains and live parts is: a loaded gun. Respect it and you will be ok and make double sure of whatever you connect to it.

[Uncle Vernon]

I'll make a couple of comment and suggestions here.

Using an RCD can be considered an essential for any form of smoke testing. And given the environment you'll likely use a guitar amp I'd consider it an essential piece of kit for gigs (do they still call them gigs) etc.  There are lots of two bit experts when it comes to music amplification and lots of interconnected equipment and poor sources of supply. It was common (and deadly) practice throughout the 60's 70's and 80's to cut the earth leads of valve amps to minimise hum loops. And there is still plenty of equipment in that state circulating.

Wall Warts are not always the best solution either given the rough environments they are likely to be used in. Drop/kick them 20 times and the bent pins loose internals and cracked cases become a safety hazard and a reliability PITA.

Given your current demand for LEDS is relatively small you may be able to use existing Amplifier power wiring to source a mains supply, but be certain of the ratings of any switches power filters etc, these will probably only be good for a few amps total and will likely be carrying near their maximum rating already.

• Double heatshrink every mains connection you make
• If soldering mains connections ensure you have a big enough iron for the likely large terminals encountered
• Heatshrink those newly soldered joints too
• use grommets and mechanical protection everywhere a cable passes through a panel and anywhere cables may otherwise chafe or rub over time.
• Earth the frame of any transformer you use and use shake-proof washers anywhere a screw pr post is used for the connection.
• Use a transformer with an electrostatic shield and an associated mains if at all possible if a transformer breaks down it will pop the fuse before contact with any ELV
• Make your earth wire the longest at any termination so that equipment will remain earthed should Any cable be pulled out due to excessive force
  
• Make sure every thing you connect to the mains is rated for mains duty. That's right 600V rated insulation for 240V mains. No insulation cut from the lids of old shirt boxes, ice cream tubs, etc, please. It may stop a flash but will not stand up to any burnout or meltdown.
  

Check and double check your work and have at least another set of (skilled) eyes check your work for you before power up. Its easy even for old hands to miss their own silly mistakes which are usually obvious immediately to another observer.   

[Chasm]

• Make your earth wire the longest at any termination so that equipment will remain earthed should Any cable be pulled out due to excessive force
  

That one is especially true when it comes to fixing power and extension cords. If something gets ripped apart the earth connection should always be the last to go.
Do not forget the pull relief either! It has that name for a reason.

[Uncle Vernon]

Do not forget the pull relief either! It has that name for a reason.

Round these parts we call those cord clamps to avoid any confusion with the Friday night pastime of lonely lads. 

[Chasm]

Well... Strain relief was to close to stain relief.

[Uncle Vernon]

touche 

[Zero999]

• Earth the frame of any transformer you use and use shake-proof washers anywhere a screw pr post is used for the connection.
• Use a transformer with an electrostatic shield and an associated mains if at all possible if a transformer breaks down it will pop the fuse before contact with any ELV

All good advice but I have to take issue with that point.

That's not applicable for toroidal transformers and is unnecessary for transformers with mains and ELV windings wound on separate bobbins.

In most cases the transformer's metal frame will be double insulated but if it's not, by all means earth it. Any modern transformer should have double insulation between the primary and secondary, otherwise the secondary should be earthed.

[Uncle Vernon]

• Earth the frame of any transformer you use and use shake-proof washers anywhere a screw pr post is used for the connection.
• Use a transformer with an electrostatic shield and an associated mains if at all possible if a transformer breaks down it will pop the fuse before contact with any ELV

All good advice but I have to take issue with that point.

That's not applicable for toroidal transformers and is unnecessary for transformers with mains and ELV windings wound on separate bobbins.

In most cases the transformer's metal frame will be double insulated but if it's not, by all means earth it. Any modern transformer should have double insulation between the primary and secondary, otherwise the secondary should be earthed.

I'd argue the effectiveness of double insulation in an unattended overload and subsequent meltdown situation, melt through ones
Even for a toroidal transformer ensuring the mounting bolt is earthed offers some protection despite it being layers of insulation away.
For fixed wiring AS3000 requires the secondary to be earthed in all situations. Transformers with an electrostatic screen (dumb description) have a metallic winding or foil between the primary and the secondary and are the only approved way of floating the secondary.

Plug in appliances escape this level of prudence so it's more often than not a tiny wrap of finest quality Taiwanese insulation is all there is between normality and possible electrocution. This also applies to many SMPS with a 2 wire cord, how many 2 metre drops can a notebook power brick sustain whilst maintaining the integrity of it's mains insulation.

Your points are valid, transformer construction does need to be taken into account when considering any safety and earthing measures necessary.

 

[Zero999]

I'd argue the effectiveness of double insulation in an unattended overload and subsequent meltdown situation, melt through ones

If you apply that logic, none of your mains equipment should be considered safe, regardless of any UL or TUV approvals.

Even for a toroidal transformer ensuring the mounting bolt is earthed offers some protection despite it being layers of insulation away.

I don't see how that offers any protection at all. If the insulation between the primary and secondary fails, the metallic blot would provide no protection at all.

For fixed wiring AS3000 requires the secondary to be earthed in all situations.

We're not talking about a fixed installation but a portable appliance. I despute that claim otherwise IT (floating mains) installations wouldn't be allowed in places like hospitals. If I remember rightly, a floating secondary is allowed but it's advisable to fit an insulation monitoring device to issue a warning in case of a fault. 12V halogen lighting is powered from a transformer with an unearthed secondary. It's known as SELV, Separated (meaning not earthed), Extra Low Voltage (under 50VAC).

And here in the UK it's mandatory for all plug sockets in the bathroom to be powered by an isolation transformer with a floating secondary.

Transformers with an electrostatic screen (dumb description) have a metallic winding or foil between the primary and the secondary and are the only approved way of floating the secondary.

Not true, all that's required is the secondary be double insulated from the primary, BS EN 60742. If it isn't you need to earth the secondary.

Plug in appliances escape this level of prudence so it's more often than not a tiny wrap of finest quality Taiwanese insulation is all there is between normality and possible electrocution. This also applies to many SMPS with a 2 wire cord, how many 2 metre drops can a notebook power brick sustain whilst maintaining the integrity of it's mains insulation.

Your points are valid, transformer construction does need to be taken into account when considering any safety and earthing measures necessary.

If the reinforced insulation can be broken that easily, it shouldn't be approved.

I'd think damaging the case would expose the user to live parts before shorting the secondary to the primary. All the insulation between primary and secondary is inside the device sealed in a plastic box which would have to be badly damaged to cause it to fail.

[Chasm]

SELV, aka Class 3

The important part about SELV is the insulation voltage. (For 230V primary voltage 2.5kV minimum.)

Important: This rule applies to all parts of the circuit.
A typical pitfall in automation system would be to use SELV to get around some rules regarding the user interface for machinery and then having the need to switch something bigger on a non SELV circuit. Simple, use a suitable relay, but: the relay has be certified for the above mentioned 2.5KV insulation voltage, or usually more since it usually will be a 230/400V net. The same also applies for all the simple stuff like switches if a panel does not exclusively consist of SELV circuits.

As you may have noticed, this drives up the cost of parts real quick.
Or, if the need for such parts and other applicable rules is not noticed, creates a /real/ liability problem once an accident happens...


That is the definition in Germany, on a quick check there seem to be different definitions what exactly SELV is in other nations.

[Uncle Vernon]

If you apply that logic, none of your mains equipment should be considered safe, regardless of any UL or TUV approvals.

None? A lot of mains double insulated equipment is not safe. Just one example. A double insulated drill with a metallic chuck and gearbox can be (and is) approved. In normal use an operator will have one hand on the metal part of the drill, yes he is double insulated from the drills cord but he is far more at risk if drilling through a live cable than if he was using a tool with a fully earthed frame.

I don't see how that offers any protection at all. If the insulation between the primary and secondary fails, the metallic blot would provide no protection at all.

Of course not, hell it's much safer to have any metal adjacent a mains winding floating from ground? </sarcasm> 

We're not talking about a fixed installation but a portable appliance. I despute that claim otherwise IT (floating mains) installations wouldn't be allowed in places like hospitals. If I remember rightly, a floating secondary is allowed but it's advisable to fit an insulation monitoring device to issue a warning in case of a fault. 12V halogen lighting is powered from a transformer with an unearthed secondary. It's known as SELV, Separated (meaning not earthed), Extra Low Voltage (under 50VAC).

Just a portable appliance, projects are only portable appliances?
The same physics apply regardless of the source of supply. Or does a mains plug offer some magic control of of all that passes through it?

Not questioning your memory but our rules which have a basis from an old copy of our rules are quite specific in terms of earthing a secondary. Nothing short of an engineers sign off and type approval allows anything different. The latter of course unfortunately can be purchased for a fee for almost any piece of imported rubbish.

Unearthed halogen lighting falls into the same categories, type approved appliance or fixed wiring out of reach. Check the sections on earthing of exposed luminaires there is no specific exclusion for ELV lamps fed from mains transformers.  The fact they generally don't get done doesn't imply any assumed safety margin.

And here in the UK it's mandatory for all plug sockets in the bathroom to be powered by an isolation transformer with a floating secondary.

That's both dumb and expensive. There are many much more effective ways of reducing the risk of electrocution in damp situations.

Not true, all that's required is the secondary be double insulated from the primary, BS EN 60742. If it isn't you need to earth the secondary.

Ah blind faith and regulations. Of course we all know that double insulation can never break down and BS-whatever will keep use safe from harm. Double insulation is not a miracle cure.

If the reinforced insulation can be broken that easily, it shouldn't be approved.

Agreed! This doesn't however make all that garbage out there in the marketplace, or any of those low cost parts shop transformers any more safe. I've seen live mains flashed across abused power bricks more times than I'd like to remember and seen more than the odd transformer meltdown/burnout.

We are primarily talking home built DIY projects here. The home constructor does not have the testing resources necessary to test and rate double insulation. It's just common sense to soundly earth anything metallic part or chassis of any project with even the remotest possibility of being touched or coming in contact with a loose wire or burnt winding.

I'd think damaging the case would expose the user to live parts before shorting the secondary to the primary. All the insulation between primary and secondary is inside the device sealed in a plastic box which would have to be badly damaged to cause it to fail.

I could show you numerous examples where this has not been the case. Badly damaged yes, but often with internal damage hidden by a battered but still intact outer casing. You've never seen plastic PCB pillars snap and come adrift within appliances?