Products > Test Equipment

Floating Scopes

<< < (6/23) > >>

Someone:

--- Quote from: dicky96 on June 22, 2017, 10:27:21 am ---Is it any more dangerous to float my Rigol 1052 and take sensible precautions such as:

Using a shrouded probe with no ground clip attached (I even have probes with shrouded BNC connector at the scope end)
Use a scope probe 'clip' so you don't need to hold the probe on the test point when powering the DUT - even solder a short wire to the test point and clip your probe to that if needs be.
Use a separate lead from floating scope ground to croc clip on floating hot ground

Than it is to use my Fluke 79 with the negative terminal clipped to hot ground while I poke around measuring voltages on the SMPS primary side with the positive probe?

Both meter and scope have no, or very few, exposed metal parts at floating potential that could be accidentally touched.
--- End quote ---
Except that the meter has been designed for this use and contains appropriate protection for faults, and more isolation from the touchable surfaces. None of that is in the scope.

dicky96:
Interesting points.  I haven't really ever looked into differential probes as my preferred method so far has been to use an isolation transformer whenever I can.

I didn't fully describe the 1980s isolated workbench environment - yes wooden benches, rubber mats etc.  Also we had strict working practices like you did not wander over and physically touch (or even distract) another technician  while he was working and you hooked up your floating scope and set the Volts/seconds before you clipped it to the DUT and powered up.  You looked at your waveforms and you didn't touch the scope while the DUT was powered.  If the settings were wrong you powered down and changed them.

With an isolation transformer on the DUT which honestly I do prefer to use guys, as some have already mentioned, danger still lurks if you don't understand that clipping your grounded scope to hot ground will no longer cause sparks or blow up the DUT/Scope but it also means you can get a nasty shock if you inadvertently touch hot live.  So in this case the grounded scope defeats some of the protection of using  the isolation transformer.  But using an isolation transformer with a grounded scope at least now it will not hurt you if you accidentally touch hot ground.  I do get this guys.

Reading the various views and replies one thing does seem to be common.  Understanding what you are doing seems to be the key.

By the way, regardless of understanding the risks andd taking the precautions I have still had a few zaps in the past 35 years but could count them on the fingers of one hand, and due to the working practices we were taught they have not been the really dangerous 'across the body' type of shock.  I can tell from experience though that a 220V 50Hz shock when I accidentally caught hold of a live linear transformer primary was bloody painful, felt like a deep throbbing and put two little black puncture holes in the heel of my thumb from the transformer terminals, and coming into contact with the collector of a running line output transistor on a CRT colour TV hurts - you can actually here it in your ears 15.625KHz. 

Both were from the DUT, I have never had a shock from test gear either floating or otherwise, and neither are something you want to experience again which increases your caution and attention level further.  So with all the precautions of whatever method you use I fully accept that working on live floating equipment is not 'safe', the best you can do is make it 'safer' and minimise the risks to an acceptable level at which you can work.

Rich

nctnico:

--- Quote from: Vtile on June 22, 2017, 09:59:00 pm ---
--- Quote from: nctnico on June 22, 2017, 09:29:18 pm ---
--- Quote from: dicky96 on June 22, 2017, 10:27:21 am ---I honestly don't think the way we were taught 'back in the day' was irresponsible or dangerous - I think we were just properly taught to understand exactly what we were doing, the risks involved and how to minimise them.  This included using shrouded probes, a separate ground connection to the chassis (not on the scope probe) and clipping the scope probe to component lead you wanted to test, then powering up the DUT.  Oh and there was nothing grounded on the wooden bench, plus it had a rubber surface mat and you stood on a rubber mat.

--- End quote ---
So you already know there is a lot more to it than just cutting the earth wire! Good, because many people think an isolation transformer is enough and happily ground the DUT again using their oscilloscope's ground lead.

The modern day equivalent to setting up an entire isolated work environment is a CAT rated differential probe. Much cheaper and much easier.

--- End quote ---
.... And the differential probe is the cheapest chinese found from ebay where the isolation can be anything a common semiconductor junction included.

--- End quote ---
This is a rather dumb remark. Note that I wrote 'CAT rated'. There are only a handfull manufacturers of differential probes and lots of rebrands (including those from Keysight, Tektronix, Lecroy, EEVblog, etc). A good differential probe starts from around $200 but sometimes good deals can be found.

tautech:
Rich
Many think when they have a scope they can look at anything.....and with using a few old-school unsafe tricks you definitely can.
There is a fault in this philosophy in that the scope only displays what it's connected to in an unsafe manner or not.
Scope users/owners need think a little differently and realise just owning a scope and a few passive probes is not the total answer to measure things safely and quickly......purchase of a scope is NOT where it ends, there are other tools needed to use a scope to anything like its potential.

Current, differential, active and HV probes are at some time needed by most for what seems to the inexperienced...........simple measurements. Like many tools, the key piece (scope) is only the start of a # of accessories to be able to fully utilise it.

I've got all except active probes and have had them for years, they all have repaid me with time savings and safety.

Stay safe guys and gals.

PA4TIM:
Tautech +1
I am in the repairbusiness too (but no consumer stuff) I most use a 500MHz FET probe, a high voltage 25 MHz diffprobe an AC+DC current probe and a Fluxgate currenprobe. Also have a low voltage diffprobe, an AC current probe, a bunch passive probes (1x, 10x, 100x) and 2 HV probes (over 10kV)

Safety first !
I never float my scopes. I have a Tek isolation guard (6901A) that I use for the DUT and behind it a homebuild very fast electronic adjustable " fuse"
Besides that I have a variac combined with an isolation transformer and a switchable series lightbulb. On my desk I have a big AC current meter and an emergency stop switch (with build in 10A resetable fuse)

I'm just in the middle of a review and teardown of an isolated Siglent portable scope.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod