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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 11th Sep 2019, 11:50 pm   #1
Jolly 7
Hexode
 
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 499
Default Earthing a part metal enclosure for mains transformer ?

I need to fit this transformer in a part-metal enclosure, which I can do without difficulty. However, for safety, I want to earth it but am not in a position to cut the metal and fit a three pin socket. Would fitting a wire to one of the transformer's mounting lugs and using a 'choccy block' to connect all three wires to another three core wire be sufficient ?
Attached Thumbnails
Click image for larger version

Name:	IMG_20190911_234229.jpg
Views:	146
Size:	79.0 KB
ID:	190015  
Jolly 7 is online now   Reply With Quote
Old 12th Sep 2019, 12:47 am   #2
bionicmerlin
Pentode
 
bionicmerlin's Avatar
 
Join Date: May 2010
Location: Warminster, Wiltshire, UK.
Posts: 170
Default Re: Earthing a part metal enclosure for mains transformer ?

There is no need to fit a socket. If it was me I would fit a 3 core flex and solder the wires live and neutral straight on the transformer and with a eye crimp on the earth wire that will fit a nut and bolt through the transformer.Make sure you scrape a little paint off on the case under the fixing screw to make sure you got a good earth connection. Ideally fit a shake proof washer.
Drill a hole through the case for the cable and fit a grommet, with a Cable tie round the cable inside to stop it pulling out or better still a small flex gland . Make sure you have a 5 amp or less fuse in the plug.
You could use chocky blocks or crimps but this way is a bit more professional.
Hope this helps Andy
bionicmerlin is offline   Reply With Quote
Old 12th Sep 2019, 12:49 am   #3
Argus25
Nonode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,282
Default Re: Earthing a part metal enclosure for mains transformer ?

Lets say mains wiring enters a metal enclosure, it is important that active is fused with an appropriate fuse and that any exposed connections are dressed with heat shrink sleeving.

In addition there needs to be a solid earth lug to the metal enclosure body, secured with a screw and lock washer and nut, ideally separate to the nut & lock washer that secures the earth lug connections onto that.

The earthing (wiring and connections) should be able to conduct at least 10A from the case body to the earth pin of the IEC connector or earth pin of the power cord's plug if there is no IEC connector. The reason for this is that if the active connection(wire) came loose in the case, or a failure that connected the active to the case, that the breaker box's circuit breaker trips . (You cannot rely that there might be a working RCD there that will trip off at a low unbalanced currents) Some authorities suggest the earthing should be 15A capable.

Never use block connectors on stranded wire where the screw end is applied directly to the wire, always use lugs or ferrules.

Just recently I did up a vintage computer project with a fair amount of discussion about a mains operated device inside a metal enclosure and the requirement of insulation and earthing that it would ideally meet, so have a read of this from about page 6 to the end with the electrical safety notes.

http://worldphaco.com/uploads/EXTERNAL_DUAL_5.pdf

A lot of cheap gear, or vintage gear comes nowhere near the standards expected today with the safety of the mains wiring. Nor unfortunately do many appliances coming out of the far east.

So you need to be sure yourself that if you do any of this mains wiring, it is beyond impeccable (and inspected) as what you make will likely fall into someone else's hands later.
Argus25 is offline   Reply With Quote
Old 12th Sep 2019, 3:20 am   #4
Terry_VK5TM
Heptode
 
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 912
Default Re: Earthing a part metal enclosure for mains transformer ?

Earthing wires should never be earthed via the transformer mounting bolts, the earth wire should have it's own bolt directly attached to the metal cabinet.

One of the reasons is that should the transformer come away from it's mount, it can leave the cabinet without an effective earth.

There are other reasons, which for the life of me I can't remember at the moment , but this is all covered in a Standard, (which again I can't remember the number of, I'll see if I can dig it up).
__________________
Terry VK5TM
https://www.vk5tm.com/
Terry_VK5TM is online now   Reply With Quote
Old 12th Sep 2019, 10:53 am   #5
David Simpson
Octode
 
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 1,919
Default Re: Earthing a part metal enclosure for mains transformer ?

I'll echo the other guy's concerns about electrical safety. Earthing requirements & safety have been the subject of many a heated thread in the last couple of years. Check "Search".
I see that the T/F is rated at 50VA(50W), so will draw approx. 200mA. Therefore your 3 pin (13A rated))plug should be fitted with a 1A fuse. 0.75mm 3 core flex would be sufficient. But, as Terry & Argus25 say, the earth conductor must go straight to the chassis. A supplementary earth wire bond can go to one of the T/F's securing bolts. 2 core flex of any type is a No No in the 21st century for any sort of mains vintage radio equipment.

Regards, David
David Simpson is offline   Reply With Quote
Old 12th Sep 2019, 11:13 am   #6
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,064
Default Re: Earthing a part metal enclosure for mains transformer ?

Earthing to a transformer fixing tab screw is better than nothing. It's the sort of thing that has been done many times before, and the world has not ended yet.

However, today's standards (which even for home use by folks like us can be regarded as good practice to abide by) says that an earth fixing must be independent of a mechanical fixing. This is so that the earth fixing has the minimum load on it, so it is less likely to come loose with vibration. And the cable must of course be slack, not tight (the official term is a 'strain relief loop').

So, you couldn't argue for instance that the left-hand bolt is the (single) transformer fixing bolt, and the right-hand one is the earth - because it clearly has the transformer hanging off it as well. If the left-hand one works loose, the weight of the transformer hangs purely on the RH one so it will soon come loose and you lose your earth, just at the time when the loose transformer has its tags touch the metalwork.

Personally, I would drill a discreet extra hole and fit a countersunk screw for a solder tag (or crimp tag) and nut.
kalee20 is online now   Reply With Quote
Old 12th Sep 2019, 12:21 pm   #7
broadgage
Octode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 1,342
Default Re: Earthing a part metal enclosure for mains transformer ?

I agree that the earth core of the mains supply flex should be directly connected to the metal case.
Earthing only via one of the transformer mountings would probably be fine in practice, but is not regarded as good practice these days.

The supply flex should be fitted with suitable strain relief and a loop or a small length of slack left in the earth core.

I would suggest a 3 amp mains fuse. The 1 amp fuse suggested in an earlier post would in my view be at some risk of blowing due to inrush current.
broadgage is offline   Reply With Quote
Old 12th Sep 2019, 1:36 pm   #8
winston_1
Hexode
 
Join Date: Oct 2012
Location: London, UK.
Posts: 300
Default Re: Earthing a part metal enclosure for mains transformer ?

Quote:
Originally Posted by broadgage View Post

I would suggest a 3 amp mains fuse. The 1 amp fuse suggested in an earlier post would in my view be at some risk of blowing due to inrush current.
Indeed. The fuse in a BS1363 plug is to protect the cable, not what is on the end of it.
winston_1 is offline   Reply With Quote
Old 12th Sep 2019, 4:36 pm   #9
David Simpson
Octode
 
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 1,919
Default Re: Earthing a part metal enclosure for mains transformer ?

There shouldn't be any significant "inrush" of current. I assume that the T/F is being used for some vintage radio pursuit. I.e. a resistive load, as opposed to a motor ? Many vintage valve radios only draw 200 to 300mA from the mains, so a 1A fuse is what I fit. That wee T/F is only rated at 50VA, so if it starts to draw 1A, 2A or near to 3A(approx. 750VA at mains voltage) - it'll seriously overheat. What is it being used for ?

Regards, David
David Simpson is offline   Reply With Quote
Old 12th Sep 2019, 4:48 pm   #10
Jolly 7
Hexode
 
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 499
Default Re: Earthing a part metal enclosure for mains transformer ?

Quote:
Originally Posted by David Simpson View Post
There shouldn't be any significant "inrush" of current. I assume that the T/F is being used for some vintage radio pursuit. I.e. a resistive load, as opposed to a motor ? Many vintage valve radios only draw 200 to 300mA from the mains, so a 1A fuse is what I fit. That wee T/F is only rated at 50VA, so if it starts to draw 1A, 2A or near to 3A(approx. 750VA at mains voltage) - it'll seriously overheat. What is it being used for ?

Regards, David
Thanks David. I plan to run a wide variety of circuits with it including a 24V DC motor. No valve radios though.
Jolly 7 is online now   Reply With Quote
Old 12th Sep 2019, 7:22 pm   #11
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,524
Default Re: Earthing a part metal enclosure for mains transformer ?

Remember that the fuse-in-the-plug is there to protect the connecting-flex, not the electrical components in the supplied device [which should have its own protective fusing according to load].

I'd fit a 3A fuse in the plug, and use flex that was rated for 5A service - that way you're 'under-fusing' to stay within the flex's ratings.

Just don't get paranoid and obsessive about this sort of stuff. Compared with the rest-of-the-world the UK is kinda crazy about fusing, conductor-ratings etc. Statistics over the last couple of decades show that in the UK the number of domestic fires is on a significantly-declining trend despite the proliferation of electrical devices.

So don't fret and worry yourself into an early grave over the nuances of fusing, flex-ratings etc.
G6Tanuki is offline   Reply With Quote
Old 12th Sep 2019, 7:51 pm   #12
turretslug
Nonode
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 2,810
Default Re: Earthing a part metal enclosure for mains transformer ?

Another argument I'd heard for using a dedicated earth screw/nut rather than an existing fastener was that someone inattentive might omit to re-instate an earth connection to a fastener that was there anyway to secure something in place after dismantling/repair, but a separate and dedicated earth screw would draw attention to the necessary presence of that earth connection. It seemed tenuous, but legislation frequently arises from mishap, sometimes tragic, and all sorts of folk end up delving into things.

50VA-size must be pretty much on the sensible limit for two-hole fixing and wrap-round clamp construction- with just two fasteners to a thin sheet-metal case, I'd be digging out penny washers for the reverse face!
turretslug is offline   Reply With Quote
Old 12th Sep 2019, 8:25 pm   #13
David Simpson
Octode
 
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 1,919
Default Re: Earthing a part metal enclosure for mains transformer ?

Well Jolly 7, If you are going to run several sub circuits off the T/F, then it would be a good idea to switch & fuse them independently off the secondary windings.
If you are planning starting the motor, as soon as you switch on, then you might well experience a significant initial surge in mains current, so yes - maybe a 2A or 3A fuse in the mains lead to the primary. Still, a 24V motor cant run off the secondary & draw more than 2A approx. from the rectified DC circuitry. In other words - 240V AC mains drawing approx. 0.2A(200mA) in the primary is the max for a 50VA rated T/F. The maths is the same for the secondary winding.
My advice would be to connect a multimeter into the pri. circuit & monitor the AC current being drawn due to your variety of circuits off the secondary.

Regards, David
David Simpson is offline   Reply With Quote
Old 12th Sep 2019, 9:27 pm   #14
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 13,446
Default Re: Earthing a part metal enclosure for mains transformer ?

Quote:
Originally Posted by David Simpson View Post
There shouldn't be any significant "inrush" of current. I assume that the T/F is being used for some vintage radio pursuit. I.e. a resistive load, as opposed to a motor ? Many vintage valve radios only draw 200 to 300mA from the mains, so a 1A fuse is what I fit. That wee T/F is only rated at 50VA, so if it starts to draw 1A, 2A or near to 3A(approx. 750VA at mains voltage) - it'll seriously overheat. What is it being used for ?

Regards, David

I disagree.... the peak inrush is determined purely by how saturated the TX core gets if switch on occurs at the worst point in the mains cycle relative to any remanent flux, the resistance of the primary winding and the primary inductance assuming a saturated core. Whilst a small TX will have much lower absolute inrush than a large one, relative to FLC it will still be "large". TX inrush current looks like exponentially decaying huge initial current spikes on top of the primary current waveform and can vary from almost nothing to hundreds of times FLC in the first cycle, depending on timing.


Motor inrush is a different beast related to stall current and how long the motor takes to run up to speed on top of any short term core saturation effects.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O

Last edited by Herald1360; 12th Sep 2019 at 9:32 pm.
Herald1360 is offline   Reply With Quote
Old 12th Sep 2019, 11:14 pm   #15
Argus25
Nonode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,282
Default Re: Earthing a part metal enclosure for mains transformer ?

Quote:
Originally Posted by Herald1360 View Post


I disagree.... the peak inrush is determined purely by how saturated the TX core gets if switch on occurs at the worst point in the mains cycle relative to any remanent flux, the resistance of the primary winding and the primary inductance assuming a saturated core. Whilst a small TX will have much lower absolute inrush than a large one, relative to FLC it will still be "large".
On a couple of points, I mentioned in post #3 about the need for two nuts and lock washers on the chassis earth stud, this also prevents anyone unscrewing the screw from the outside if the head is exposed.

For power supplies of equivalent power rating, in the case of switching on a transformer primary across the mains power the peak inrush current will always be lower than switching on a switchmode PSU (without inrush current mitigation circuitry) because in the latter case you are applying the voltage directly (via the rectifiers with have a very low internal Z), from the mains source (a very low internal Z) to discharged capacitors which by their nature oppose a change in applied voltage, and it could be longer for them to get to the peak voltage than for the transformer's flux field to get established and the current to drop (if the timing of the power switch on was at zero crossing, encouraging a high peak initial primary current) and there is the primary resistance present in the transformer case to help limit the peak current and its not insignificant in small power transformers.

In most cases with transformer power supplies, you can work out the running power and current, and use a slow blow fuse 1.5 times that value or a fast blow fuse 2 x that value and not get nuisance (non fault related) fuse blows.

This is not the case for the switchmode psu (without inrush mitigation).

The method I use to select the fuse, (as outlined in the article I mentioned that has a 65W psu with no inrush mitigation) is to have the supply normally loaded, start with a fast fuse, with a low fuse value that blows at turn on. Increase the fuse value in steps until it survives at least 3 to 5 turn on attempts. Then use a fuse twice that current rating if a fast blow or 1.5 times that rating if a slow blow. In the long run, if it is sailing too close to the wind, the fuse you have selected will fatigue & fail after a few hundred turn on cycles, and you might need to increase one step up in current rating. Its all a much easier arrangement if the switchmode psu has inrush mitigation. Most if not all desktop computer psu's have it.

So what I'm getting at here is that if you compare two mains operated power supplies of equivalent power output, one a transformer, the other a swichmode psu , without inrush mitigation circuitry, mostly you will find the switchmode unit has a higher fuse rating to withstand the inrush currents.

Last edited by Argus25; 12th Sep 2019 at 11:20 pm.
Argus25 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 10:41 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 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.