using test equipment in an ungrounded outlet house - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: August 17, 2015, 04:44:15 pm
content: A guy who bought a pinball machine from me is using it in a 1950s house with two prong outlets (no third dedicated earth ground).He needs some help debugging a problem with a kicker coil and associated power transistor drive circuit, and I offered to sell him my old Rigol 50mhz (100mhz) scope and show him how to use it.Obviously I would prefer he had traditional three prong outlets with earth ground but he says that is not possible. What precautions would you take/recommend? I doubt he has an isolation transformer of large enough capacity to run the pinball (I have a 5kW unit but it is around 90 pounds and I don't want to lug it over there). I was thinking to just have him always test the voltage between the scope probe ground lead and the place on the pinball circuit where he wants to connect scope ground to make sure they are at the same potential (hopefully they would be at/near the neutral AC line which would be tied to earth ground somewhere in the house breaker box)Suggestions?

author: electr_peter
author_rank: Unknown
timestamp: August 17, 2015, 06:13:15 pm
content: Please do not try to do nonsensical "earthing" solutions via connecting earth wire to a pipe nearby or similar. Not good enough and creates a myriad of bigger dangers this way, etc.If nothing better is available, use this method:2 hole socket <- portable RCD plug/socket <- power cord extension with at least two sockets <- scope + machineHowever, scope and mains powered equipment do not mix well, there are many caveats. YouMUSTtreat every metal surface/metal on the pinball machine and scope (most importantly, BNC connectors) asLIVEuntil 100% proven otherwise.Pinball machine should have a power isolation transformer inside - if it does not have it, STOP measuring - it is too dangerous with a regular scope and probes. Special isolated probes with isolated oscilloscope channels are required in such cases.If pinball machine has a transformer inside, you can measure isolated low voltage side with a scope (both "isolated" and "low voltage" are important).Also, scope maybe too much for the problem at hand. A proper multimeter with isolated probes should be more than enough for 95% of possible problems.

author: KTP
author_rank: Frequent Contributor
timestamp: August 17, 2015, 06:43:35 pm
content: Thanks.I also believe 95% of the problems can be solved with a multimeter (heck, a lot of them with the pinball off). Checking transistors, wiring, bad solenoid coils.)Then there are some things that a scope really would help with, like investigating glitches which may be caused by noise on the data lines. Also some of the solenoids do not fire for long enough to register on a multimeter and a scope lets you see the pulse and just gives you a good idea that the circuitry is performing as it should.This particular pinball is notorious (Gottlieb Haunted House) for problems and poor engineering, although I do believe it has a main transformer (will verify that when I go over there).I figure an extra safety check would be to at least verify the surfaces of the pinball machine are not carrying any stray voltage relative to earth ground or the neutral AC line. What I didn't want him to do is connect the oscilloscope ground to the isolated pinball machine ground after the transformer and then for some reason that ground on the scope wasn't really ground.

author: electr_peter
author_rank: Unknown
timestamp: August 17, 2015, 07:08:19 pm
content: If the guy in question does not have electronics/electrics background or experience, I strongly suggest not to give him a very dangerous, capable and misleading tools for a dangerous job. Mostly for his safety, then for your responsibility, then for your tools, then for the health of pinball machine.Thus, the easiest solution - recommend a person which can help with repair.In case buyer is proficient in electrics and electronics and just lacks some tools, then situation is completely different.

author: KTP
author_rank: Frequent Contributor
timestamp: August 17, 2015, 09:09:51 pm
content: He has an EE degree but hasn't really touched electronics for a number of years (that makes me kind of sad for some reason).

author: vk6zgo
author_rank: Super Contributor
timestamp: August 19, 2015, 06:00:17 am
content: If the pinball machine has a normal isolated transformer power supply,theoutputof that supply does not normally have any direct connection to the Mains,except in the following manner:-One side of each supply may be (probably is) connected to "chassis ground".The "chassis ground"may or may notbe further connected to theMains Earthif you were using a 3 prong connector.In any transformer type power supply,it will not be connected to either the Active or Neutral conductors of the power cord.In a switchmode supply,the only connection back to the primary side of the high frequency transformer used is via fairly low value capacitors.If an Oscilloscope probe "earth clip" is placed on the "chassis ground",& all voltages are measured with respect to that point,there is no hazard in normal circuitry.Unless the person working on the machine starts messing with the primary side of the transformer,or in a switchmode,the circuitry prior to the secondary of the high frequency transformer,they will not get into trouble.Some very old equipment,(mainly early TVs,& some Radios) used a "transformerless supply" where the "chassis" was connected to the Neutral line (if you put the plug in correctly),& the Active line directly rectified.These were "ex-factory",heavily insulated ,but over time,the insulation would get damaged,leaving the device as a "death trap".Both transformer type supplies and modern SMPS are light years away from those things in terms of safety.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 19, 2015, 06:10:32 am
content: I would recommend the first step is to retro-fit the gadget with a proper 3-wire mains power cord with proper protective ground pin.Then, at least you can plug the gadget and whatever test equipment into a power strip that joins the protective grounds together. That at least gives you a common, local point of reference whether the wall mains outlet has a proper protective ground available or not.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 19, 2015, 06:28:47 am
content: This may be helpful:http://arcadecontrols.com/files/Manuals/Black%20Hole%20-%20Haunted%20House%20Repair%20Guide.pdf