UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 21st Oct 2021, 5:48 pm   #1
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default 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.
6SN7WGTB is offline  
Old 21st Oct 2021, 6:11 pm   #2
winston_1
Hexode
 
Join Date: Oct 2012
Location: London, UK.
Posts: 497
Default 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.
winston_1 is offline  
Old 21st Oct 2021, 7:57 pm   #3
jamesinnewcastl
Heptode
 
jamesinnewcastl's Avatar
 
Join Date: Jul 2009
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 722
Default Re: AC Millivoltmeter - earthing question

Quote:
Originally Posted by winston_1 View Post
..... You should not earth double insulated equipment.........
Hi Winston - very interested in your statement. Why should you "not" earth double insulated equipment? Lots of double insulated equipment is plastic cased so it would be pointless but I can't see why adding an earth wire is not allowed. What problem would it produce?

I ask because I tend to go belt and braces and for me popping an earth to any exposed metal is usual.

Cheers
James
jamesinnewcastl is offline  
Old 21st Oct 2021, 9:04 pm   #4
GMB
Dekatron
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
Default 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.
I used to do that, but now I am not so sure. Obviously it must be a proper double-insulated device to not have an earth.

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).
GMB is offline  
Old 21st Oct 2021, 10:29 pm   #5
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default Re: AC Millivoltmeter - earthing question

It's not DE.
6SN7WGTB is offline  
Old 22nd Oct 2021, 9:57 am   #6
DMcMahon
Dekatron
 
DMcMahon's Avatar
 
Join Date: Apr 2019
Location: Worthing, West Sussex, UK.
Posts: 6,587
Default Re: AC Millivoltmeter - earthing question

[QUOTE=GMB;1416258]
Quote:
Obviously it must be a proper double-insulated device to not have an earth.
If I read that statement correctly then I do not agree, i.e. lots of vintage equipment is not double insulated and only have 2 core mains cable with no earth wire.

David
DMcMahon is offline  
Old 22nd Oct 2021, 11:03 am   #7
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default 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
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline  
Old 22nd Oct 2021, 11:08 am   #8
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default 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.
6SN7WGTB is offline  
Old 22nd Oct 2021, 1:24 pm   #9
dave cox
Nonode
 
dave cox's Avatar
 
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,059
Default 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
dave cox is offline  
Old 22nd Oct 2021, 6:55 pm   #10
GMB
Dekatron
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
Default Re: AC Millivoltmeter - earthing question

It appears my statement has been unclear so let me restate it:

Quote:
it must be a proper double-insulated device to not have an earth.
I.e. if a device does not have an earth then it must be a proper double insulated one, or it is unsafe. Not having an earth does not necessarily make it double insulated! The question for a vintage device is whether it is appropriated constructed but perhaps pre-dates the legal definition of double insulated of modern times. That's a hard question to answer, so I would assume the worst.

(Sorry for being a bit off topic)
GMB is offline  
Old 22nd Oct 2021, 7:50 pm   #11
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default 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.
6SN7WGTB is offline  
Old 22nd Oct 2021, 7:53 pm   #12
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default 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"
6SN7WGTB is offline  
Old 22nd Oct 2021, 7:55 pm   #13
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default 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.
6SN7WGTB is offline  
Old 23rd Oct 2021, 11:17 am   #14
Trigon.
Hexode
 
Trigon.'s Avatar
 
Join Date: Jun 2018
Location: Buckinghamshire, UK.
Posts: 382
Default Re: AC Millivoltmeter - earthing question

Quote:
Originally Posted by 6SN7WGTB View Post
"Double insulated power tools were invented by Spencer Rees in November of 1969."
Although that statement can be found all over the internet, it's not actually true.

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:
Another approach to the problem of accidental electric shock has been to provide portable electric power tools, or appliances with an additional dielectric barrier, or the power tool or appliance is more commonly referred to as being "double insulated.” This has been done, for example, by using a housing of dielectric material, such as plastic, to completely enclose the “live' parts of the motor. Alternately, plastic liners or bushings have been used to isolate the "live' parts of the motor from those which the operator normally comes in contact with.
Cheers
Trigon. is online now  
Old 23rd Oct 2021, 6:48 pm   #15
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default Re: AC Millivoltmeter - earthing question

Interesting - thanks for the correction.

At least my other quotes were from the manuals in front of me...!
6SN7WGTB is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:07 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.