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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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21st Oct 2021, 5:48 pm | #1 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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AC Millivoltmeter - earthing question
Just acquired a rather nice Kikusui dual channel AC MV.
It (a) a 240V-labelled model and (b) has the original mains cable which is TWIN core. So, there is no protective earth. The chassis is a steel/aluminium and plenty exposed. The two channel inputs are UHF-type sockets with an additional ground terminal. The two pre-amp outputs are 4mm banana binding posts, red and black. Each channel 'ground' (the UHF shroud, the gnd terminal and the black output post) is actually connected to the chassis via what measures as 47Ω. (so channel to channel ground measures 94Ω). There were at least THREE versions of this AC voltmeter, with very similar numbers (a 1831, an 1831A and a 1831B model). I quote from the manuals: (A model) This had a switch on the front which EITHER floated/disconnected the channel grounds from the chassis OR "connected to the case ground with respective resistors of which resistances are sufficiently low as compared with the input resistance". (it's 1MΩ input impedance) (B model) No switch. But "the outer conductor and GND terminals are electrically connected to the meter panel and chassis" So I have a no switch model with the A model via resistor grounding mode. I've spoken to Kikusui and they have no manual for my model (the A and B are on their website). Can anyone explain (a) why no PE, and (b) why I shouldn't earth the chassis when I rewire it? Yes, I could of course create an A model by adding a switch but my most pressing issue is the PE. |
21st Oct 2021, 6:11 pm | #2 |
Hexode
Join Date: Oct 2012
Location: London, UK.
Posts: 497
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Re: AC Millivoltmeter - earthing question
Maybe it is double insulated. You should not earth double insulated equipment.
Electronic equipment is often not earthed for the simple reason if you start taking measurements on live equipment you could do damage. |
21st Oct 2021, 7:57 pm | #3 | |
Heptode
Join Date: Jul 2009
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 722
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Re: AC Millivoltmeter - earthing question
Quote:
I ask because I tend to go belt and braces and for me popping an earth to any exposed metal is usual. Cheers James |
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21st Oct 2021, 9:04 pm | #4 | |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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Re: AC Millivoltmeter - earthing question
Quote:
Thanks to the growing popularity of PME, the old mains earth is not perhaps as safe as it used to be in some situations (hence all the equipotential bonding that is now required). |
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21st Oct 2021, 10:29 pm | #5 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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Re: AC Millivoltmeter - earthing question
It's not DE.
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22nd Oct 2021, 9:57 am | #6 | |
Dekatron
Join Date: Apr 2019
Location: Worthing, West Sussex, UK.
Posts: 6,587
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Re: AC Millivoltmeter - earthing question
[QUOTE=GMB;1416258]
Quote:
David |
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22nd Oct 2021, 11:03 am | #7 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
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Re: AC Millivoltmeter - earthing question
Sometimes it's useful for the chassis of test equipment not to be earthed, in order to avoid introducing earth loops when doing sensitive measurements. This is a legitimate reason to disconnect the safety earth from a piece of equipment such as a millivoltmeter. I've done it myself when making measurements in a "noisy" environment, such as when heavy currents are being switched nearby. It's absolutely fine when the equipment is used in a controlled environment with awareness of what's going on. To add extra confusion, there exist instruments with an earthed metal case but a separate floating chassis inside, insulated from the case. This is precisely to prevent earth loops. My HP 334A distortion analyser (which coincidentally is also an AC millivoltmeter) is made this way. However, it's an expensive belt-and-braces way to make equipment.
Maybe the Kikusui was designed with this in mind. It may be too old to be a proper "double insulated" (class 2) appliance. Such an appliance has reinforced and supplementary insulation such that no single fault could make any external metalwork live. Without examining the Kikusui it's impossible to say whether it meets this requirement. However, this doesn't necessarily mean that it's dangerous, or conversely that earthing it would be a bad thing. There is no safety risk involved with earthing a metal-cased Class 2 (double insulated) appliance. However, it's a modification to the original specification, so on your head be it, and be prepared to take responsibility for the modified appliance. There is some really horrible cheap vintage equipment around of US or Japanese origin which has no proper mains transformer and has one side of the mains (hopefully the neutral!) connected to the exposed metal chassis. This can be extremely dangerous, but such equipment is very rare in my experience in the UK. Summarising, there are two possible reasons not to earth the chassis of the Kikusui: 1. You want to make high precision measurements avoiding earth loops. Assuming the instrument is in good condition as originally manufactured, you'll be fine. Check the insulation resistance with a megger or similar to be sure. 2. It's so nasty and cheap that the case is connected to one side of the mains. In this case, I'd suggest cutting the mains lead off and leaving the instrument in a glass case somewhere as a warning from history. Otherwise, you can earth it if you like, with no safety risk. Indeed, if the same instrument exists in a version with an earthed case, that's a good indication that it's designed to be connected that way. Chris
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22nd Oct 2021, 11:08 am | #8 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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Re: AC Millivoltmeter - earthing question
It's a REALLY well made device, like all of (at least that) vintage Kikusui - each bolt paint sealed, all wiring looms dressed and tied every inch or so.
OK, I am inclined to add the safety earth (although I appreciate and agree with Chris' comments about sensible use of non-earthed equipment), and probably a switch to emulate the A version option to lift channel grounds if needed. |
22nd Oct 2021, 1:24 pm | #9 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,059
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Re: AC Millivoltmeter - earthing question
As it has a separate GND connection, connected to the chassis, I would expect / demand that the separate GND connection was connect to a safety earth. Being able to control if the signal GND is connected to the safety earth is a useful feature
It was likely made before the strict rules on 'double insulated' and the manufacturer made the pragmatic decision that the instrument could be used in an environment lacking a safety earth facility. The older Kikusui kit is usually built to a very high standard dc |
22nd Oct 2021, 6:55 pm | #10 | |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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Re: AC Millivoltmeter - earthing question
It appears my statement has been unclear so let me restate it:
Quote:
(Sorry for being a bit off topic) |
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22nd Oct 2021, 7:50 pm | #11 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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Re: AC Millivoltmeter - earthing question
I suspect the semantics are getting in the way here.
Yes, according to current regulations an appliance which is not Class II constructed does require a PE. However, I think what many have observed is that older equipment may be any of: (a) not having a earth and very much needing one, (b) not having an earth, not being truly compliant with Class II but without undue hazard under most reasonable circumstances under supervised and sensible use, or even, (c) not being at all compliant with anything but conceivably meeting the spirit of Class II (All American 5 radio sets maybe!!). As it stands this unit I'm referring to is (b) having reviewed it, but I agree fully that it needs a PE, and so it's getting one. I will try to design my modification to allow me to lift the chassis from the channel ground, but maintain the PE. As a last resort I do have a device (used for testing of certain audio equipment) that goes inline with the mains and allows me to temporarily lift the PE if I so choose to do so, but that's my risk. Thus I won't be installing a mod that offers this functionality in the equipment. |
22nd Oct 2021, 7:53 pm | #12 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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Re: AC Millivoltmeter - earthing question
We are way off topic, but
"Double insulated power tools were invented by Spencer Rees in November of 1969. Many other inventions have been patented by power tool manufacturers making the double insulated tool even safer" |
22nd Oct 2021, 7:55 pm | #13 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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Re: AC Millivoltmeter - earthing question
Further still, don't forget that equipment can now be classed as "Class II FE" meaning that it is double/reinforced insulation but carries a PE for "functional or screening" reasons.
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23rd Oct 2021, 11:17 am | #14 | ||
Hexode
Join Date: Jun 2018
Location: Buckinghamshire, UK.
Posts: 382
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Re: AC Millivoltmeter - earthing question
Quote:
His patent simply improves the use of the by then aleady well established concept of 'double insulation' to the motor construction in power tools with metal bodies. From the patent background section:- Quote:
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23rd Oct 2021, 6:48 pm | #15 |
Hexode
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
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Re: AC Millivoltmeter - earthing question
Interesting - thanks for the correction.
At least my other quotes were from the manuals in front of me...! |