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Transformer safety

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Dr.EM

New Member
Would this transformer be safe to operate without earthing the secondaries at all?

**broken link removed**

The EN60742 rating seems to imply it has stronger insulation than usual and can take more overloading without meltdown.

I do use one of these without secondary earth (I implemented one, then quickly removed it after it caused numerous problems when connecting it to other equiptment), though I don't imagine that is totally safe, it's only me that uses it and i'm aware of that fact:

**broken link removed**

But the first one is more suitable to be operated without earthing? It provides isolation under just about any circumstance? I can't find much out about the safety ratings ever.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Any mains transformer sold legally in the UK (and those you list all seem to comply to various standards) are perfectly fine not to earth the secondary - it's not something I would ever consider looking for to be honest!.

The only problem with NOT earthing the secondary is that a high static voltage might accumulate on the secondary side metalwork. This is why you normally have a resistor and capacitor combination between the secondary chassis and the incoming mains - these components need to be specifically rated for this use.

In your case, you could always just connect a high value resistor between chassis and mains earth, as there's nothing to stop you using a three core lead. The resistor will prevent any static build up.

You might also have a look at the power supply at **broken link removed** which uses a bridge rectifier and 10 ohm to prevent hum loops - I've never tried it, but supposedly it works?.
 

Dr.EM

New Member
Oh right, well thats good. I didn't know it was strictly ok to not earth at all, probably because of that westhost site actually since he seems to suggest doing it, but then thats Australia and mabye it's different there.

If I used any transformer in a metal case, then I would earth said metal case (via no resistance). That doesn't mean my secondary is connected to the case though as any electronics ground/secondary ground connection will not be in contact with the case. I'd use insulated phono connectors etc so I can use star grounds where appropriate. But your saying I should earth the electronic/seconday ground via a large (1M?) resistor so the ground on the phono connector or similar can't build up a charge? I see your point; if the unit was connected to another earthed unit though, that charge couldn't build up in the first place (which is the main reason for my wanting to avoid earthing to eliminate associated earth loops).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
That's pretty well it - Australia is probably no different to the UK, it all depends what you are doing, and what you're connecting to what. Certainly for a guitar combo you really NEED the chassis earthing - as you're holding a nice guitar connected to it in your sweaty hands!.

The thing with earth loops is to understand what causes them a simple safe method is to break the ground connection on the input sockets, it's common to use a switch (called a ground lift switch) to do this.
 

Dr.EM

New Member
I remember you mentioning the earth lifing, but when I tried it it was even worse doing it that way? The buzzing got louder with the increasing volume instead of remaining low all the time.

It's here he seems to make a point of earthing the 0v secondary line;

**broken link removed**

where as you say it is safe not to do so with those transformers? I would definately earth any metal chassis and that doesn't seem to cause any problems, but it's the 0v line that can.

EDIT: Also, how is it that the bridge in the breaker circuit doesn't pass any current except in the event of a fault? Because the voltage is nominally too low to overcome the diode drops in a normal earth loop so the current flows through the 10ohm resistor instead?

Oh yeah, another thing. My own bought amplifier (Rotel RA01) is in a metal case, yet it can't possibly have any earth since it connects via a 2 pin inlet. How is it that is "safe"?
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Dr.EM said:
I remember you mentioning the earth lifing, but when I tried it it was even worse doing it that way? The buzzing got louder with the increasing volume instead of remaining low all the time.

You must have done something wrong somewhere, but it's obviously hard to know without seeing it.

To be honest though, I've had no earth loop problems for years! - using my Behringer mixer/amp, with keyboard, bass and guitar amps DI's into it (and NOT transformer isolated DI's). Although I must admit the keyboard isn't earthed anyway - it's class II with a two core lead, but everything else is class I.

It's here he seems to make a point of earthing the 0v secondary line;

**broken link removed**

where as you say it is safe not to do so with those transformers? I would definately earth any metal chassis and that doesn't seem to cause any problems, but it's the 0v line that can.

EDIT: Also, how is it that the bridge in the breaker circuit doesn't pass any current except in the event of a fault? Because the voltage is nominally too low to overcome the diode drops in a normal earth loop so the current flows through the 10ohm resistor instead?

Presumably so? - as I said, I keep meaning to try it.

Oh yeah, another thing. My own bought amplifier (Rotel RA01) is in a metal case, yet it can't possibly have any earth since it connects via a 2 pin inlet. How is it that is "safe"?

It's a class II appliance, double insulated, no earth required - as much modern electronics is. Like I mentioned earlier, it will have specially rated components connecting the metal work to the mains to prevent static build up. This does mean though you can feel it's 'live' as it gives a tiny leakage current from the mains.
 

Dr.EM

New Member
The amp was connected to my PC which is earthed. The amp is too, so using ordinary connection, I experienced some buzzing. To earth lift, I tried firstly disconnecting the shield at the amp input, simply by pulling the phono plug far enough away that only the pin touched and not the shroud. This caused the described problem. I then tried disconnecting the shield at the PC by placing a small bit of tape around the ground band on the 3.5mm plug. I got the same result. Transformers eliminated the problem nicely, but are expensive for good ones (I was suprised how acceptable LT44's sounded mind).

Let me know if you do try the earth breaker, not sure if it is actually legal here? Don't know how you'd find out.

How would you go about making a class 2 appliance then? You'd presumably require a special transformer, which those Rapid ones are not? I can't feel any charge on the amp, but it collects a lot of dust :)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Dr.EM said:
How would you go about making a class 2 appliance then? You'd presumably require a special transformer, which those Rapid ones are not? I can't feel any charge on the amp, but it collects a lot of dust :)

You seem to have an obsession with 'special' transformers?, the Rapid ones should be fine - most class II audio systems use really cheap and nasty Chinese transformers. But unless you understand EXACTLY what you are doing, you probably shouldn't be looking at making class II devices - and it would also be a good idea to PAT test then if you did to check their safety.

As for your amp, if anything plugged into it is earthed, then the amp is as well!.
 

Dr.EM

New Member
I thought there would need to be a specific approval that means the transformer could not go live at its secondaries even in the event of a total meltdown? Again, this is from the westhost site. I have wall warts with plastic earth pins, the transformer inside them (although cheap) couldn't just be any old one, it would need to be a specially rated one?

I know what you mean about the connected equiptment making them earthed, but again from the westhost site, he say's that interconnects and audio connections arn't likely to be heavy duty enough to carry fault currents safely. I expect they might do, if a fault occurs, the current is only there till a fuse blows and stops the flow anyhow?

If I use mains transformers again for audio, I expect I will use those encapsulated ones, earth the chassis and implement the earth loop breaker for the secondary/0v connection. I really can't see much wrong with that; many people use old pieces of equiptment which have not been checked recently and may not conform to todays standards, yet I don't suspect many (any?) recieve shocks. The amp I made is current powering speakers for my electric drum kit monitoring, since that runs from an un-earthed wallwart it causes no issues.
 

Hero999

Banned
Nigel Goodwin said:
You seem to have an obsession with 'special' transformers?, the Rapid ones should be fine - most class II audio systems use really cheap and nasty Chinese transformers.

It has nothing to do with cost, it's more to do with transformer design. Double insulated transformers are always wound on double section safety bobbins whist class I designs the primary and secondaries are all wound on a single section bobbin. I have quite a large transformer which has all the primary and secondaries all on one bobbin and if you touch a neon lamp on one of the secondaries it will glow dimly, I have done this test on class II transformers and the neon doesn't light.

Having said most modern transformers are double insulated, especially those found in mains adaptors and other class II equipment.

All of the above applies to transformers with lamimated E-cores. I don't know about torroidial types, it might depend on the degree of insulation between the primary and secondaries. I would look at the datasheet,m try Googling for any safety codes or ask the manufacturer for more information. To make sure you could always try the neon lamp test I described above.
 
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Dr.EM

New Member
Interesting, thats what i'm talking about, class 2 transformers. These ones specifically say they are:

**broken link removed**

Which means you can leave off earthing the secondary altogether? The Rapid ones don't say class 2 anywhere, but have that same EN60742 rating.

I guess these might be, since they say "doube bobbin" construction? E cores don't often fit into the slim cases I like to use, but mabye as an external powercube type supply.

**broken link removed**
 

grim

New Member
Hero999 said:
Double insulated transformers are always wound on double section safety bobbins whist class I designs the primary and secondaries are all wound on a single section bobbin.

All of the above applies to transformers with lamimated E-cores. I don't know about torroidial types, it might depend on the degree of insulation between the primary and secondaries.

ahha the old mix up between DOUBLE insulation and DOUBLE section bobbins.:D

double insulated transformers are NOT wound on double section bobbins.

some definitions from EN61558 will probably help.

There are 3 classes of isolation transformer.

Class I - protection by earthing. Has BASIC insulation and a protective earth, to prevent electric shocks.

Class II - does not rely on BASIC insulation, instead uses DOUBLE or REINFORCED insulation

Class III - only generates low voltages.


we are interested here in Class II transformers, the ones with the double square logo (one inside the other)

BASIC insulation - provides basic protection from electric shocks

SUPPLEMENTARY insulation is applied in addition to BASIC insulation, as a back up if PRIMARY insulation fails

DOUBLE insulation - comprises of BASIC insulation and SUPPLEMENTARY insulation

REINFORCED insulation - single insulation system equivilant to DOUBLE insulation.


in the real world, BASIC insulation would be a couple of layers of insulation tape. To REINFORCE it a third layer would be applied, making the transformer DOUBLE insulated. This applies to both SINGLE bobbin coils and torroidals.
Usually the primary is applied, then the three layers of insulation, then the secondary.

Double section bobbins, only have the one piece of insulation between primary and secondary, but it is many times the thickness of the tape a double insulated part would use, so is classed as REINFORCED insulation.

The other consideration is not through the insulation, but around it. Creepage and clearance distances are the real problem with single section double insulated transformers.

A Class II REINFORCED double section bobbin gets around this by having a shroud which fits over the bobbin, and locks into a tongue in the central wall, creating a long path between primary and secondary.

A complete novice can make a safe transormer in seconds with this method.

hope this clarifies things a bit;)
 

Dr.EM

New Member
Thanks for that, you work with transformers looking at your sig?

What would you say about either of the transformers in my first post, the toroids? Niether specifically say class 2, so presumably arnt? Very few do actually, but I expect a lot are since you say it is easy to do. Mabye they are but haven't had the official approval?
 

grim

New Member
yeah, I've been designing these things for years:D

ONE of them is absolutely definately 100% a double insulated one. The other is made by a company I don't work for, so I can't say, but I expect it is too;)

There is no need to earth the outputs at all.

I appreciate the space saving a torroid offers, and they are perfectly good at what they do, but if the insulation between primary and secondary was critical, then I would prefer a double section laminated transformer.

The insulation on a torroid is polyester film tape. electrically it's brilliant, until you over heat it, then it crinckles up and all but disapears. Additionally the tape has to be applied to the transformer, and anything that has to be applied can be applied wrong - a doubel section bobbin just exists, it doesn't rely upon an operator/machine to do the job propelry. obviously tyhey are tested, and 99.9999% of the time there won't be a problem. a nylon bobbin tales care of the 0.0001% there is

If the transformer was to power a train set, and my son was holding the output, then i would use a laminated part, not a torroid.;)
 

Hero999

Banned
Thanks for the information.

My understanding of the safety standards is that if there's a possibility the insulation could fail then it should be class 1 not class 2 so I'm suprised that the torroidial transformers pass.

grim said:
Class III - only generates low voltages.

To be picky it's SELV which is defined as below 120VDC or 50VAC and isolated from earth. Low voltage means below 1kVAC or 1.5kVDC.

If I remember rightly the type of insulation also depends on the area of intended operation as well as the voltage. For dry areas no insulation is required for voltages below 60VDC or 25VAC, for other areas it's below 30VDC or 12VDC and in a very wet area like a bathroom all live parts must have at least basic insulation, or can voltages bellow 15VDC or 6VAC be uninsulated? I can't remember now, I'll have to look it up.
 

Dr.EM

New Member
I wouldn't have any worries about touching a 15vdc supply and have done before with very sweaty hands (ie, + in one, - in other). Probably done 30vdc too actually at some point. Remember getting a slight tingling from a live phantom powered (+48v) circuit, but that is supplied by 6.8k or something anyway.
 

grim

New Member
well of course it's always possible for insulation to fail. get anything hot enough it will fail, get a high enough voltage it will fail.

so the standard sets out the working conditions for the insulation - the temperatures and voltages it has to withstand.

so a 230:12v torroid with a 40'C rise will be fine with the three layers of tape.
and then protect the transformer from external overloads, the only thing that can really cause it to overheat.

but if the customer replaces the fuse with a nail, and doubles the load on the output, a torroid could fail with the loss of insulation between primary and secondary - a nylon double section transformer would fail with a short circuit primary
 

grim

New Member
yes they are [thumbs up]
 

Dr.EM

New Member
Good :)

I had a look inside an old 6VA transformer I have (I think I burnt it out a while ago). I can see the doube/split bobbin now and yes, it looks like a very safe system. Would take a bit of melting to get those windings to touch each other! The "fully shrouded" feature of the TruPower ones presumably adds a nylon covering over the coils as well?
 

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