earth ground  safety - Page 1
Forum section: electronics / projects

author: vangheli
author_rank: Contributor
timestamp: November 09, 2021, 09:58:30 pm
content: Hi everybody.I request help about earth ground connections.I have some analog oscilloscopes (Hameg and Tektronix) . In all of them ,except one, the chassis is connected to the yellow-green wire of the standard 3 wire socket of the local mains (220V) , which is connected to earth ground. So , I expect ,in case of failure inside the oscilloscope, that the differential switch will trip, or in any case that the path to earth-ground is less resistive than my body.Well, examining one of the oscilloscopes and precisely an Hameg307 version 1, I noticed that the chassis ,functioning as reference for all the internal circuits , is not connected to the yellow-green wire of the earth-ground system. I was thinking of a modification introduced by the previous owner, but a check on the schematics of this model , confirmed that the scope was actually designed like that. The yellow-green wire is internally connected to the armature of the transformer which generates, with multiple secondaries ,all the voltages required.I was not able to check the continuity between the armature and the yellow-green wire ,because the armature has a heavy external layer of isolating paint.Strangely enough , all the following version of the Hameg 307 (2,3 and4) have the chassis connected to the earth ground .Here are my questions :-is this connection to the transformer armature safe for the user ? (the chassis is floating)-is there any advantage in this type of design?-if I weld the green-yellow wire to the chassis , will I jeopardize the functioning of the scope ?Thank you for your help!

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2021, 10:15:50 pm
content: Have you measured the continuity between the ground pin on the line cord and the shell of an input BNC connector on that 'scope?The core of the transformer should be grounded to both the chassis and the line cord. When I was employed, we were required to ground PE wires to the chassis or framework independently of any screw fasteners that held anything else (e.g., mounting screw for a transformer).

author: vangheli
author_rank: Contributor
timestamp: November 09, 2021, 10:48:37 pm
content: Hi Tim. Thanks for your answer.Yes , I did the measure you are talking about: while there is continuity between the chassis and the external part of the bnc connector , there is no continuity between the bnc and the ground pin in the socket (this is the reason of my worry).I checked again the schematic : a wire connects the core of the transformer to the chassis (I didn't actually see this wire on the real scope, perhaps it is internal to the core and not easily seen). So the chassis and the shell of the bnc's should be connected to the ground-earth wire of the line chord through the transformer core , but this is not the case actually.I suspect there is a bad connection , if always the core of the transformer and the chassis should be connected per safe standard practice.

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2021, 11:14:48 pm
content: In my belief, the PE wire of the line cord should go to the chassis directly, and have continuity to the BNC shell.It's not clear why the core of the transformer should be insulated from the chassis, which is what your measurement implies.

author: vangheli
author_rank: Contributor
timestamp: November 09, 2021, 11:23:46 pm
content: Tim ,I see your answer , just after modifying my post : please comment the modified post . Thank you

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2021, 11:27:31 pm
content: The core-grounding indication on the schematic might not be a wire--it might indicate grounding through the mounting screw.If that connection is bad, you can ground the third wire directly and see why the transformer core is not grounded properly.Sometimes, a "star washer" on the screw is needed to make a good ground connection to something like a transformer core.

author: vangheli
author_rank: Contributor
timestamp: November 09, 2021, 11:38:42 pm
content: Actually they indicated a black wire for the connection between core and chassis (they use the symbol ground reference) . I was not able to see this black wire: I will check again tomorrow (now here it is late night).Is there ,in your opinion, the possibility to harm the scope , if I connect the chassis to the earth wire? Thank you for your comments.

author: TimFox
author_rank: Super Contributor
timestamp: November 10, 2021, 02:25:36 am
content: In my opinion, based on what you said here, I would connect the third wire (PE) to the chassis and make sure that the transformer core is also connected to the chassis.

author: richard.cs
author_rank: Super Contributor
timestamp: November 10, 2021, 09:42:51 am
content: Does this scope have any labelling saying "double insulated" "class II" or have a symbol that looks like a square within a square? Is the transformer wound on a split bobbin with a rigid barrier between primary and secondary, or one on top of the other? It could be entirely legitimate that they are not connected, in which case I would expect the manual to specify a maximum allowable voltage between signal ground and mains earth.

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2021, 12:41:35 pm
content: It won't be Class II if it is fitted with a 3 core mains lead (or 3 pin IEC connector) - that's not permitted.If the BNC shells are connected to the chassis and this it touchable, then it is susceptible to dangerous elevation if a probe clip accidentally touches a high voltage rail.I suspect that the mains earth connection is either broken, or has been disabled by a previous owner.I suppose it might be a manufacturing error but that seems unlikely. [EDIT: if the schematic shows the earth going to the transformer frame then I assume that this would be intended to be connected to the chassis via the mounting bolts, maybe the paint was too thick.]

author: richard.cs
author_rank: Super Contributor
timestamp: November 10, 2021, 01:17:00 pm
content: Hybrid designs using a mix of class II and Class I techniques are allowed, as are class II devices with the earth connection used only for EMC not for safety*, though I think you are right that they shouldn't be marked as class II. It's hard to tell without being able to see it, but I still think it may be intentional given the schematics. In any case adding in a chassis to mains earth connection will not be damaging.*Technically the earth wire should then be cream coloured as it is a functional earth not a protective earth, but everyone always ignores this.

author: Zero999
author_rank: Super Contributor
timestamp: November 10, 2021, 01:18:09 pm
content: Quote from: Gyro on November 10, 2021, 12:41:35 pmI hope it would be designed so that can't happen, i.e. the paint being removed, or star/crinkle washers used to bite through the paint and the earth tested, as part of the manufacture process.It's possible it happened later, when someone services it, i.e. replaced the transformer, or stripped it down and repainted it.Quote from: richard.cs on November 10, 2021, 09:42:51 amAttached is a picture of the symbol, for the benefit of the OP.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 10, 2021, 02:43:59 pm
content: Before the emergence of 'scopes with isolated channels for power electronics, a dubious work-round was to "float the scope".This involved disconnecting the earth ground somewhere so the whole scope would float up to the bnc probe common connected to a unit under test. ( For example measuring gates on the uppers of a H bridge)Maybe this was done on the Hameg and you haven't located the point of disconnect.

author: vangheli
author_rank: Contributor
timestamp: November 10, 2021, 03:29:03 pm
content: Hi Richard, thank you for your contribute. I wish also thank all the other friends who intervened on this subject.About your questions:-there are no labels at all-I cannot say if the transformer has a rigid separation between primary and secondaries (this detail is not visible from outside)Some facts:-the core is connected to the chassis (I positively checked it) and this connection is through the connecting bolts and I assume also through the black wire ,also reported on the schematic, entering the bobbin frame on the bottom ,and welded on the PCB (see pictures): I think this wire is connected internally to the armature-the bnc shells are connected to the chassis but not connected to the earth wire (yellow-green) of the power chord-there is a cream colored wire connected with the earth wire through a welding on the PCB : this wire enter the bobbin frame on the bottom , come out on the upper part but it is connected to nothing (see pictures)I hope not have been confusing things

author: richard.cs
author_rank: Super Contributor
timestamp: November 10, 2021, 04:16:27 pm
content: The photos help a lot. That's not a split bobbin transformer, and nor is there any evidence of class II construction. The PCB tracking reveals that the design is very old, 1970s perhaps. I would therefore say that this should be a class I device, with user-accessible parts connected to mains earth.

author: vangheli
author_rank: Contributor
timestamp: November 10, 2021, 04:27:03 pm
content: Quote from: richard.cs on November 10, 2021, 04:16:27 pmSo , if I understand correctly your thinking, it would be safer if I connect the chassis to mains earth?Thanks

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2021, 07:35:11 pm
content: Yes it would be far safer, I think many/most people would say essential, after looking at the construction, to connect the chassis to mains earth.Now the method...Properly, you should disconnect the earth wire from the PCB and connect it directly to a hole in the chassis, using a screw, solder tag and shake-proof washers to ensure a long term secure connection. You can then run a jumper wire from this chassis connection back to the original earth point on the PCB. It looks as if you have enough wire length on the mains cable to do this, if not, you may need to re-terminate. In the first photo, it looks as if there are already some chassis holes that you can use.The alternative (tempting) method would be to simply run a jumper wire from the existing PCB earth wire termination to a mechanical joint on the chassis. This wouldn't be correct under current wiring standards however - The chassis dedicated earth connection should be the first destination for the earth wire.

author: Greybeard
author_rank: Frequent Contributor
timestamp: January 03, 2022, 09:55:35 pm
content: Quote from: vangheli on November 09, 2021, 09:58:30 pmCan you post the complete manual/schematics of your HM307-1, please?I know for sure that Hameg produced some oscilloscope models usingdouble isolated and shielded transformers, such as HM512-4 (1973) to HM512-8 (1980), HM412-2 (1976) to HM412-4 (1978), HM312-6 (1976), HM812-2 (1978).Maybe other Hameg models within this time frame followed the same grounding philosophy.Quote

author: Gyro
author_rank: Super Contributor
timestamp: January 04, 2022, 09:43:51 am
content: Unfortunately, while that may have been the intention in the less safety focussed days when those scopes were designed and manufactured, it is not acceptable these days, for a couple of reasons...1. From the OP's photos, the internal mains wiring most certainly doesn't meet the present day requirements of a Class II double insulated product.2. It is no longer permissible for the metal chassis and front panel to be allowed to become live if the probe ground clip accidentally contacts live parts.Whatever the original intention, it is no longer safe to put into use in that manner, just as many old appliances may need updating to be safe to use today. Therearescopes today that are suitable of floating use, but they don't have exposed metalwork that is connected to the probe ground.

author: Greybeard
author_rank: Frequent Contributor
timestamp: January 04, 2022, 01:43:46 pm
content: For my undestanding the old "double isolated" way is still safe enough, if the user thinks twice before connecting the scope.In shorting case live wire could short to PE-shield layer between primary and secondary winding(s) and trip the RCD. It could not short to a secondary winding because several isolation layers and first shielding (PE) are between primary winding and secondary. If you make sure, the connections between mains cord, mains switch and primary winding are safe, I don't see a problem. VDE also did not see a problem these days.If using a scope with chassis connected to PE, you also have to think twice.It's just the choice to potentially destroy the scope or to electrocute the user...Or you install a circuit (neon bulb), that detects and indicates high voltage between the chassis and PE."hands off indicator"