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View Full Version : The Case of PRAT & the Cooker



monya
22-07-2010, 14:02
Got a new dual cooker delivered today - the old one, gas, was plugged into the mains for the display, timer etc with 13 amp connector plugged into the kitchen ring circuit. The new one, when connected up tomorrow has the benefit of a new 32 amp run to the cu.
Thing is, I unplugged the old one this morning so it could be taken away and later sat down for a few toons. I noticed something was up after 30 seconds. A cliche but sounded like a veil, which I didn't know was there, had been lifted. The music was just more... foot tapping, involving - that sort of stuff.
The wall socket for the stereo has its own cable running direct to the cu as well which I thought would eliminate interference. Obviously not. So can mr Sparks remove the system cable from the cu where it has its own socket, and wire it up somehow to bypass the cu? Wire it directly into the house feed somehow. If so, what should I specify he does?
Any clues folks?

Reid Malenfant
22-07-2010, 14:55
IMO i doubt you'd find this worthwhile for a couple of reasons. 1. You'd need to install a second consumer unit with RCD to comply with electrical regulations as they stand today (as far as i'm aware). 2. It'd be rather expensive :(

Why not install a filter between the cooker & the socket feeding it? IE prevent the interference from getting back into the mains :) Very similar to dealing with interference caused by alternators & ignition systems in cars, you have to treat the cause rather than attempting to get round it.

Good luck with whatever you do ;)

Ali Tait
22-07-2010, 17:17
You could try running your system with balanced mains.This would remove any interference.

DSJR
22-07-2010, 17:38
A separate audio feed via a separate small RCD as mark suggests would be a good idea regardless I think. If the separate feed to the Hifi is already there, it would simply be a matter of removing the wires from the main CU, installing a RCD (?) and connecting straight to the meter output, thereby bypassing the main CU and cooker feed.

Ask your electician when (s)he comes.

Reid Malenfant
22-07-2010, 18:03
You could try running your system with balanced mains.This would remove any interference.
:lol: I have no idea why this didn't occur to me as i'll soon be installing a PS Audio Power Plant which outputs balanced mains. However the same could be done with a 240 - 120 0 120V transformer as you mention. Some fitering components would improve things still further.

A separate audio feed via a separate small RCD as mark suggests would be a good idea regardless I think. If the separate feed to the Hifi is already there, it would simply be a matter of removing the wires from the main CU, installing a RCD (?) and connecting straight to the meter output, thereby bypassing the main CU and cooker feed.

Ask your electician when (s)he comes.
I'm still not convinced this would make much difference simply because you'll only be moving the point that the hifi system is connected to the mains by a few feet. The interference that the (odd) cooker was sticking out isn't really going to be attenuated much over this distance i wouldn't have thought.

It might be worth a try, but as i mentioned earlier this will cost money & as things are nowadays only a qualified electrician can do this. Think £20 - £30 for small consumer unit & £50 for an hours work or more for something that might make zero difference :scratch:

As long as only a clock is being powered on the cooker a small mains filter costing about £3 - £5 wired in between the cooker & socket would be my first port of call. It's cheap & it should do the trick much better than moving a connection a few feet & praying (IMO) :)

monya
22-07-2010, 18:06
Thanks for the advice. Balanced mains sounds pricey but will ask about a separate RCD to the meter output. Maybe now the offending item has gone the new one may be better supressed if regs are tighter now. Must say balanced sounds tasty. May have a listen sometime but not aat Russ Andrews prices

Ali Tait
22-07-2010, 18:14
:lol: I have no idea why this didn't occur to me as i'll soon be installing a PS Audio Power Plant which outputs balanced mains. However the same could be done with a 240 - 120 0 120V transformer as you mention. Some fitering components would improve things still further.

I'm still not convinced this would make much difference simply because you'll only be moving the point that the hifi system is connected to the mains by a few feet. The interference that the (odd) cooker was sticking out isn't really going to be attenuated much over this distance i wouldn't have thought.

It might be worth a try, but as i mentioned earlier this will cost money & as things are nowadays only a qualified electrician can do this. Think £20 - £30 for small consumer unit & £50 for an hours work or more for something that might make zero difference :scratch:

As long as only a clock is being powered on the cooker a small mains filter costing about £3 - £5 wired in between the cooker & socket would be my first port of call. It's cheap & it should do the trick much better than moving a connection a few feet & praying (IMO) :)

You could also try an isolation trannie cheaply by looking on ebay for the trannies used to supply 110v on sites.Buy two and connect the secondaries together,so you get 240 out.

Reid Malenfant
22-07-2010, 18:25
You could also try an isolation trannie cheaply by looking on ebay for the trannies used to supply 110v on sites.Buy two and connect the secondaries together,so you get 240 out.
Ah, slight misconception here. They aren't isolation transformers, they are autotransformers & simply step the voltage down to a level that is less likely to kill you if you get electrocuted. The secondary is literally the primary tapped off at the 110V point & the Neutral being standard mains Neutral. In that instance you could never connect the secondaries together as you'd effectively be shorting out the mains :eek: I might be wrong on this score & they indeed may well be full isolation transformers in which case accept an apology now ;) However i don't think they are...

You need proper isolation transformers & not autotransformers to do this trick. If you find a pair then yes, you could ground the centre of the secondaries & get 100% balanced mains :)

Ali Tait
22-07-2010, 19:14
No,the secondary is center tapped,giving 55v WRT to earth from each phase.

Reid Malenfant
22-07-2010, 19:33
In that case they must be isolation transformers & could be used to do the trick with a little modification :) I'm not overly familiar with them so it's great to hear from someone who knows more than myself.

Cheers for that information, may well come in handy some time ;) I can see me hunting down a couple of 3KW units to feed my power amps balanced mains in the future2279 They are cheap enough if you play your cards right...

Ali Tait
22-07-2010, 19:51
No worries,I'm not that familiar with them myself,despite using them all the time! I do know however that the 110 output is center tapped so you get a max of 55v to earth from each phase,thus making the use of power tools safer.

Ali Tait
22-07-2010, 19:53
Found this on a site selling them-

Input 240 volts 50Hz single phase. Complete with 2 metres of 3-core flexible cable to BS6500 for connection to mains supply and a 13 amp plug.
Output 110 volts. Centre tapped and earth screen between primary and secondary windings which is bonded to the centre tap of the secondary winding, the earth pin on each output socket and earth core of the mains input plugs and sockets.

Ali Tait
22-07-2010, 19:59
I found this a while ago,they will do you a balanced 120-0-120 for the same price if you ask-

http://www.airlinktransformers.com/transformer/wm3602-industrial-transformer.asp

Reid Malenfant
22-07-2010, 20:10
Gotcha ;) In this case you'd need to move the earth to the end of the 55 0 55V taps & connect the second transformer with the earth to the opposite end of the windings so both windings are in phase & earths at the same potential.

That would work :) Balanced mains ;)

Ali Tait
22-07-2010, 20:20
:)

Steve Toy
26-07-2010, 11:14
If I've understood the OP correctly you've got a dedicated line with a shared CU for your cooker and your hi-fi has a dedicated line also to the shared CU.

Here I've got a dedicated line from the main CU to the cookr and an entirely separate CU and dedicated line for the hi-fi. It sounds great for it but oddly when the fridge kicks in and out my system still picks it up. :confused: :scratch:

Stratmangler
26-07-2010, 11:20
Here I've got a dedicated line from the main CU to the cookr and an entirely separate CU and dedicated line for the hi-fi. It sounds great for it but oddly when the fridge kicks in and out my system still picks it up. :confused: :scratch:

Go to Maplin and get part CP76H
Connect directly across the live and neutral in the plug for the fridge.
It should reduce the problem, possibly to nothing.
A ferrite clamp on the fridge cable might also help.

Barry
26-07-2010, 12:01
If I've understood the OP correctly you've got a dedicated line with a shared CU for your cooker and your hi-fi has a dedicated line also to the shared CU.

Here I've got a dedicated line from the main CU to the cooker and an entirely separate CU and dedicated line for the hi-fi. It sounds great for it but oddly when the fridge kicks in and out my system still picks it up. :confused: :scratch:

Hi Steve,

If the distance between fridge socket and the consumer unit is short and of a similar distance to that of the hi-fi spur, then it is quite possible for the thermostat clicks to be transmitted through the mains wiring.

Another way the clicks could be picked up, is if the fridge is on the other side of the wall from your equipment.

Either way the best cure is to attack the source of the problem: that of the fridge thermostat. A 0.1uF 400V Mylar capacitor fitted across the contacts should do the trick. Ask your local friendly electrician to do this for you.

Trust this is of some help.

Regards

Ali Tait
26-07-2010, 12:16
This is why I wonder at people saying they get an improvement fitting a dedicated CU for their system.After all,it's still connected to the original CU,so any mains rf noise will still get through.Another,more expensive solution would be to go to a balanced supply Steve.This should prevent any noise getting there at all.At least,all I know who have tried it have been pleased with the results.

Steve Toy
26-07-2010, 12:34
There is still an improvement from dedicated mains. Subjectively lower noise floor, more oomph etc like improving amplification and the dedicated earth is the icing on the cake - like improving a source component.

Ali Tait
26-07-2010, 13:56
Lower noise floor how exactly? It's still the same mains.

Reid Malenfant
26-07-2010, 15:25
Lower noise floor how exactly? It's still the same mains.
Ah yes, however the hifi won't be on a ring along with whatever is intefering. Admittedly it'll all join together at some point, though this point will generally be of lower impedance due to much thicker cables.

You'll likely still get some kind of dross from noisy gear but it will be lower than if you happened to be on the same mains ring ;)

Barry
26-07-2010, 16:36
Ah yes, however the hifi won't be on a ring along with whatever is intefering. Admittedly it'll all join together at some point, though this point will generally be of lower impedance due to much thicker cables.

You'll likely still get some kind of dross from noisy gear but it will be lower than if you happened to be on the same mains ring ;)

It's not so much a matter of reducing the (source) impedance: you're not interested in 50Hz, what you are intersted in is supressing, or attenuating noise spikes and other high frequency hash caused by SMPs and RFI, i.e. 500kHz to 1MHz.

In supplying the hi-fi system from a dedicated spur back to the consumer unit, you are increasing the cable path length between the source of the noise and the audio gear. The noise signals have to travel back to the CU and then out again to the audio system. In lengthening the cable path you are increasing the series inductance, and hence impedance to high frequency signals. It is this that attenuates the noise.

Improving the (a)earthing and (b)using unbalanced/balanced transformers adds icing on the cake, by offering a lower impedance to ground (a) and self cancellation to common mode noise and RFI (b).

Ali Tait
26-07-2010, 17:05
Yes,that's why I'd tend to favour going balanced over the other methods.Probably work out cheaper too.

Reid Malenfant
26-07-2010, 17:08
Yes, i agree with that wholeheartedly apart from one thing ;)

Your equipment will be most interested in 50Hz :) Generally most people will install cable that is generally a heavier gauge on a dedicated audio spur to reduce line fluctuations due to current draw from large amplifiers.

What you said is what i was attempting to get at though, the fact that the noise producing artical is on a seperate ring & has a long way to travel to the hifi kit.

Barry
26-07-2010, 17:23
Yes, I agree with that wholeheartedly apart from one thing ;)

Your equipment will be most interested in 50Hz :) Generally most people will install cable that is generally a heavier gauge on a dedicated audio spur to reduce line fluctuations due to current draw from large amplifiers.

What you said is what I was attempting to get at though, the fact that the noise producing artical is on a seperate ring & has a long way to travel to the hifi kit.

In lengthening the cable you will increase the inductance pro rata. In using a heavier gauge wire you will reduce the inductance, but by a lesser amount than it increases with length. Overall the inductance is increased. The series impedance created is proportional to the frequency. Hence the increase in impedance to the noise is something like 10,000 to 20,000 times that presented to 50Hz.

Increasing the gauge of the wire will, as you say, reduce the source impedance and the voltage drop. This means that the mains supply is 'stiffer' and more resistant to voltage fluctations as the load varies.

Regards

Reid Malenfant
26-07-2010, 17:27
Yes,that's why I'd tend to favour going balanced over the other methods.Probably work out cheaper too.
I'm seriously thinking about this myself right now ;)

After out little discussion a page or so back i just thought i'd check out the prices of site transformers. Looks like i could get a brand new pair of 3.3KVA transformers delivered for just under £100 :)

I'm going to take a look inside one though before being daft & spending money, pretty sure my neighbour uses one for his job so i'll ask if i can have a look & take some measurements :whistle:

Barry
26-07-2010, 17:32
I'm seriously thinking about this myself right now ;)

After out little discussion a page or so back I just thought I'd check out the prices of site transformers. Looks like I could get a brand new pair of 3.3KVA transformers delivered for just under £100 :)

I'm going to take a look inside one though before being daft & spending money, pretty sure my neighbour uses one for his job so i'll ask if i can have a look & take some measurements :whistle:

Do tell! - Are these 240V primary / 2 x 120V secondary transformers? £100 for a 3.3kVA rated device seems remarkably cheap.

Regards

Reid Malenfant
26-07-2010, 17:48
Do tell! - Are these 240V primary / 2 x 120V secondary transformers? £100 for a 3.3kVA rated device seems remarkably cheap.

Regards
Hi Barry, not exactly no. From what Ali said i gather that they are 55 0 55V secondaries with the 0V connected to earth. The proposal is to disconnect the earth on the secondary 0V & connect earth to one end of the secondary & do the same with transformer number 2 but connect earth to the opposite end of the winding (IE the other 55V tap at the opposite end of the winding to transformer number 1).

What you'll then have is 110 0 110V balanced mains. Obviously if the secondary is 120V (or 60 0 60V) it'd output 120 0 120V. This shouldn't be too hard to do, well definately not for me as i have designed & built transformers & SMPS in the past (part of my previous & previous to that employment).

Two transformers would be need though to do this & i have found some at just under £50 each delivered.

Of course this is only conjecture & depends on if Ali is correct & that they are isolation transformers. I always thought they were autotransformers but you live & learn :)

Bests..

Ali Tait
26-07-2010, 18:24
I may well be wrong! Buut,I thought the whole point was the safety aspect,in that an earth fault would result in someone getting a 55v shock as opposed to a 110v.

Ali Tait
26-07-2010, 18:43
Well I just got my trannie out of the van and measured the 110v sockets.From earth to each phase gives 60v.Phase to phase gives 120v.Question answered.

Reid Malenfant
26-07-2010, 18:43
I may well be wrong! Buut,I thought the whole point was the safety aspect,in that an earth fault would result in someone getting a 55v shock as opposed to a 110v.
We'll find out soon enough my friend, i'll pester my next door neighbour as soon as i see his van outside :donk:

:lolsign:

Reid Malenfant
26-07-2010, 18:46
Well I just got my trannie out of the van and measured the 110v sockets.From earth to each phase gives 60v.Phase to phase gives 120v.Question answered.
Cool, please let me confirm this & i'll happily blow £100 on 6.6KVA of balanced mains supply 2336

E2A:- The only thing that bothers me is for safetys sake we'd need to employ fuses on both the Live & Neutral leads as effectively both would be live, i can live with that though. With double insulated kit (no earth) it wouldn't be needed...

Reid Malenfant
27-07-2010, 13:20
Yes i can confirm exactly the same thing :) Borrowed mateys' site transformer & checked continuity from output earth to mains primary earth & then checked the secondary voltages in relation to that - 57 0 57V at this end.

I'm off to buy some transformers ;) Total price of two x 3.3KVA delivered is £94.95 :scratch: If anyone wants a link click this (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160460411265&ssPageName=STRK:MEWAX:IT) :) I can see yet another project coming on, balanced mains here i come :lolsign:

Ali Tait
27-07-2010, 15:11
Yep,though I'd fuse both legs anyway,just to be safe.Post pics and detailed explanation of the mods,so people wanting to do this can do so safely.There could be an issue with possible fault currents with this though.It needs to be implemented carefully.

monya
27-07-2010, 16:18
Years ago I used a Trichord Research transformer between the wall and the distribution box. I thought it made a big improvement for 18 months until I removed it when PRAT returned again. So I'm not convinced on those things. Mr sparks thought it would be nuts to seperate the spur to another cu saying ' a few feet won't make any difference at all. Perhaps he's right.

Reid Malenfant
27-07-2010, 16:21
Yep,though I'd fuse both legs anyway,just to be safe.Post pics and detailed explanation of the mods,so people wanting to do this can do so safely.There could be an issue with possible fault currents with this though.It needs to be implemented carefully.
I'm more interested in fusing both Live & Neutral in any equipment that isn't double insulated. If it's earthed it'll need both of them fusing for safety reasons.

Probably a good idea to fuse the output from the balancing transformers on both legs as well as you mention :) I'm sure you'll see a project sooner or later ;)

Steve Toy
27-07-2010, 16:46
Not very WAF friendly I know :rolleyes: but there is a case for having the dedicated CU well away from the main one which is usually in the garage or a behind a lockable steel door outside and in the same room as your kit (the living room in my case) :

http://img375.imageshack.us/img375/1986/g1000hdconnectors009.jpg (http://img375.imageshack.us/i/g1000hdconnectors009.jpg/)

Ali Tait
27-07-2010, 16:59
Mark,can you post a drawing of exactly how you will connect the transformers?

Reid Malenfant
27-07-2010, 18:07
Mark,can you post a drawing of exactly how you will connect the transformers?
Yep, just wait for them to get here & i'll see about knocking something together ;)

Ali Tait
27-07-2010, 18:36
Ta!

Reid Malenfant
30-07-2010, 13:09
Just to let you know that the pair of transformers have arrived ;) I popped the lid off of one & immediately started cursing as the transformer had been potted up, on further reflection it makes no difference at all except it keep the thing quieter :lolsign:

I'll get a thread together elsewhere on here so you can see what i'll be doing :)