Safety Equipment for home repairs - Page 1
Forum section: electronics / beginners

author: sdancer75
author_rank: Frequent Contributor
timestamp: October 10, 2022, 07:56:18 am
content: Hi,What safety equipment should I use for my home repairs when I use hot wires to make my measurement tests?The only equipment I currently use is a residual current device (RCD) relay in the main board that is distributed to all my home.I mostly repair SMPS as my own hobby, so I am not a professional. I heard about isolated transformers but it's difficult to find and professionals concluded that they are not so safe. What do you recommend? I live in Europe.Best Regards,

author: Messtechniker
author_rank: Frequent Contributor
timestamp: October 10, 2022, 09:03:55 am
content: A megger like this one for example: Peaktech 2695 (insulation tester and continuity).I now use it on all electrical equipment and cables coming into the houseafter a friend was injured quite severely by a faulty portable floodlightfor use on building sites.

author: sdancer75
author_rank: Frequent Contributor
timestamp: October 10, 2022, 11:08:40 am
content: Quote from: Messtechniker on October 10, 2022, 09:03:55 amThanks, how this can protect me in case I touch a hot wire?Regards,

author: inse
author_rank: Super Contributor
timestamp: October 10, 2022, 11:41:06 am
content: For repairs on live equipment, an isolation transformer is definitely recommended.

author: Berni
author_rank: Super Contributor
timestamp: October 10, 2022, 11:58:19 am
content: Yep you want a 1:1 isolation transformer and a "light bulb current limiter".The isolation transformer helps that you can only get shocked by touching two points in a circuit and it makes using oscilloscopes a lot easier as there is no risk of blowing the whole thing up by sticking the ground clip in the wrong spot. They also make short circuits a lot less violent since the transformer is so small that it can only provide 10 or 20 A even into a dead short. Without the transformer a dead short will draw however much the resistance of the house wiring allows and this can be >1000A.The light bulb current limiter is good for when you expect a short to happen. It is just a incandescent light bulb in series with the live wire. This limits the current down to 100s of mA so that way if a short happens there is no damage at all. So you can power it up with the light bulb in series, once you see there is no short then close a switch that jumpers live past the bulb for full mains power.The one thing to watch out is that 1:1 isolation transformers make RCD protections useless since the transformer is hiding the ground fault currents from it. But you should not rely on a RCD saving you anyway, working with mains will always be dangerous, so you have to pay attention to what you are doing. Minimize the amount of conductive metal around your work area, one hand rule etc...

author: sdancer75
author_rank: Frequent Contributor
timestamp: October 10, 2022, 01:10:02 pm
content: Quote from: Berni on October 10, 2022, 11:58:19 amExcellent, can you recommend a trusted isolation transformer?

author: strawberry
author_rank: Super Contributor
timestamp: October 10, 2022, 01:13:56 pm
content: isolation transformer will do nothing for 240VAC human safety perspective (modern oscilloscope test lead is earth/neutral grounded. electroboom)clutters on workbench could be dangerous , especially for beginners ( it is hard task to follow live wires and focus on task at same time )incandescent light bulb or resistor to limit destructive current ( will save diode bridges, PCB traces , fuses, ..)discharge HV capacitorsall transformers are not equal. pri;sec capacitance ..

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 12, 2022, 08:34:31 am
content: Quote from: sdancer75 on October 10, 2022, 01:10:02 pmI guess there isn't a large choice. You buy what you can find in your area. Something like several hundred Volt-Ampere rated should work (depending on the typical devices you work with).For example, I have 400 VA rated transformer, which suits me ok with all my audio devices and PC ATX power supplies.

author: Berni
author_rank: Super Contributor
timestamp: October 12, 2022, 09:16:19 am
content: Yep for a 1:1 transformer it is best to first look at local used markets. They are heavy to ship around but are common enough that you might find one around.You can also make a 1:1 transformer out of two similar transformers. So you turn 220V to 12V with one and then use another in reverse to turn 12V into 220V.It is also possible to use a 3 phase transformer as a single phase 1:1 transformer. But those will typically be way too big for what you need. You actually want a small transformer in the low 100s of VA range because they are not capable of sending a large amount of current into a short circuit because of how small they are.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 13, 2022, 05:12:02 pm
content: Real protection comes from knowing what you're doing.The insides of an smps will be able to deliver high voltage at high current irrespective of any external kit.So they are bloody dangerous if you forget where the hazzards lurk. A number one piece of gear is a cap discharger.Its just a bank of power resistors in a box with a couple of test leads. See Mr Carlsons discharge doobrey.Make sure that the DVM you have has the right CAT rating.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: October 17, 2022, 02:28:02 am
content: Correct for all the above suggestions.However, some isolation transformers are not true isolation. study up on it if buying used, open it up and make sure it is. I bought one used opened it up and found some dingbat and added a jumper wire across the two transformers.You can get iso transformers up to 500v.Another dumb suggestion. use one hand ..the "keep one hand behind your back." statement. and make sure no other of your body is grounded? with smps not so easily, so using clips and clamps to do with one hand?re-high voltage. I have worked on some HV tube amps and SeeMr CarlsonsYT channel has been a great help

author: bdunham7
author_rank: Super Contributor
timestamp: October 17, 2022, 03:06:40 am
content: An isolation transformer is a good part of a strategy to work on these safely, but I would suggest avoiding any poking or prodding at all while the SMPS is energized. You can set up your measurement power-off and then do your testing hands-off. Nothing good happens putting test probes into a high energy circuit while it is live. 99% of the time you won't electrocute yourself if you make a mistake, but you will damage your circuit and probes when you short stuff out unintentionally.An RCD or iso-transformer won't protect you from energized capacitors and those can hurt. When things blow up with the cover off, the flying parts can hurt. As the old Honda ATV ads said "stupid hurts".

author: crgarcia
author_rank: Regular Contributor
timestamp: October 17, 2022, 08:20:41 am
content: Pretty much the same situation as you, and also looking into how to protect myself from myselfI bought an isolation transformer (link)I use that to protect my oscilloscope, but it does not protect me.I also have a 2A din-mount breaker, as most of the things I do do not require a lot of currentI was also thinking in building some board with very fast blown fuses, that I can replace for the amount of current I expect to be using for each project

author: Berni
author_rank: Super Contributor
timestamp: October 17, 2022, 11:14:46 am
content: Careful with using a 1:1 transformer to float an oscilloscope. This can make the whole scope case live.Fuses don't protect your device under test very well. When a direct short happens they will still let trough a huge peak current before the fuse burns out and breaks the circuit. If you power your DUT over a 1:1 transformer you get isolation and at the same time limit the current it can take to safer levels.If you make a short directly from the wall you get a loud bang and a shower of sparks. But if you make a short on the output of a small isolation transformer you just get a quiet pop, maybe a little spark and an angry buzzing transformer.

author: crgarcia
author_rank: Regular Contributor
timestamp: October 17, 2022, 12:06:46 pm
content: Quote from: Berni on October 17, 2022, 11:14:46 amWhy is that?In my case, first, I want to protect myself, far after the scope, and in the very last the rest of the equipment

author: Berni
author_rank: Super Contributor
timestamp: October 17, 2022, 12:46:12 pm
content: Quote from: crgarcia on October 17, 2022, 12:06:46 pmThe ground of all the BNC connectors on a oscilloscope is connected to mains earth. This makes sure that any exposed metal on the scope is safe to touch.Once you disconnect the earth on an oscilloscope then you can clip the ground on your probe to any voltage you want, however this also brings all exposed metal on the scope to that potential as well. So if you float your scope, clip a probe ground to mains live, now all the exposed BNC connectors on your scope also have live on then and will give you a shock if you touch them. This is particularly dangerous if you have an older scope with a metal housing, that whole housing will become live too. Especially dangerous since people commonly assume test equipment to be safe to touch.So it is generally safer to isolate the DUT. This way you get shock protection from touching any single part of it while you can still hook one scope probe ground to anywhere you want (but making the rest of the DUT dangerous to touch). In general you should avoid that anyway.The scope won't really be any safer because of being isolated. Especially since a perfectly working scope wont cause faults on the mains side. But you might as well use the isolation transformer at the DUT to protect it better in case something goes really wrong in there.

author: strawberry
author_rank: Super Contributor
timestamp: October 17, 2022, 04:25:52 pm
content: anything that makes you think you are safeearth leakage is dangerous where moisture is present (wet basement...defective rubber boots)in normal condition earth leakage is not dangerous (heart stimulator is exception)if you can get better isolation 240V / 1Mohm = 240uA (phase indicator screwdriver)rubber mat and rubber/silicone gloves is safe

author: sdancer75
author_rank: Frequent Contributor
timestamp: October 18, 2022, 06:13:52 am
content: It would be great to create a manual guide from your professional experience on what to avoid and the right equipment for all of us that we are not so experienced. It would be nice also to attach this guide to any forum that concerns electronics. We are people that want to learn from you and then in consequence teach other people around us. Thank you to all !!