Earthing/Grounding Single Phase Transformer Secondary Winding Neutral

You are wrong, or I am not able to follow your thoughts properly. Don't, repeat please don't do that. The reason being, you do not know the reason. Follow the adage: Let slipping dogs sleep!

"Let sleeping dogs lie" Joshi.

Thanks Lyn. I struggled for 15 minutes for that word. Just could not get it. I had to submit the post, so there I am again at it. You are a true friend. Door will always be open for you Lyn. You don't have to knock, just walk-in.

Do not attempt this untill you are absolutly sure there are no stray earth faults on the system.

Well, if one of the phases is connected to earth and you do the neutral as well, you'll make something go BANG, Guvnor!

The lack of a neutral ground link on the secondary side may be deliberate.ie an isolated output.

We have no idea what this arrangement is servicing so cannot judge.

first thing you have to understand that the Earth is alternative the Neutral in the transformers and any device if you want to do Earth connect the body of the transformer to the ground and leave the Neutral .

The earth conductor is there to provide a return path for the fault current wile the fault exists, i.e. until the circuit protective device(s) operate(s) so as to disconnect the fault. The neutral conductor is there to carry the imbalance current back to the transformer centre-tapping. So they cannot be alternatives, as they do different jobs.

PW, that is exactly right but there will be no earth fault current if the transformer secondary winding is fully floating, i.e. if it has no link to earth. A fully floating system has no earth fault protection although it may have overload protection.

GA and well explained too....

The neutral conductor is there to carry the imbalance current back to the transformer centre-tapping

Only in the US. No centre tapped neutral in most other places.

Anywhere where single phase loads are on three phase transformers (most of the known world?), the current drawn from the phase returns via a neutral wire in the cable to the switch box.....(as its AC, its actually flowing in both directions at either 50 or 60 HZ, but its easier to understand how I wrote it!)...

What current is drawn from the phase is/should be! identical to that in the neutral wire to that point, EXCEPT when you have a fault current returning via the earth connection...if an ELCB or similar is fitted, a fault current above a preset level will trip the breaker for that circuit. I personally have all 3 phases with the German equivalent of ELCBs, till now nobody has been shocked, nor have they tripped.

I once tripped one at work, it HURT!!!!.....

If you make a simple drawing, you will see exactly what I mean about neutral current flow in single phase ONLY situations.........

I know all that.

The arrangement where the secondary is a single phase centre tap is an american thing.

What I think op has is a 230 to 100 single phase step down transformer without a centre tap and he wants to know whether or not one leg of that secondary needs to be decked or not. The answer is it depends on what the 100V load is.

Dear Wal,

you must have come late to the party and maybe you haven't read all the previous posts fully, I covered this in detail several posts ago.....please read back....

Also you wrote:-

The arrangement where the secondary is a single phase centre tap is an american thing.

Which is either bad English or a misunderstanding of the method, or both, but I cannot say which, but a possible correct sentence could be as follows (note spelling and the usage of capitals as well!):-

The arrangement where the mains is a single 220VAC phase with a center tap used as a neutral/earth connection is an American thing.

Actually, many countries around the world, usually ones either under US influence, or ones that were under US influence, have this type of home mains setup. I do believe that even Canada has the same sort of arrangements.....

I personally don't find the US system to be any real improvement on the European system, for example, there are just as many deaths from 110VAC as there are from around 220VAC (nominally 240VAC)......

This lower voltage just effectively doubles the amount of copper needed to get the same amount of power through, an extra cost. Also cables for portable devices are often less flexible for the same reason....

The 60Hz is a slight improvement on 50Hz, but 100Hz (or more) would have been even better.....

Not having 3 phases into the house has also frustrated many US DIYers wanting to use a powerful something or other.....welder for example.....

We in Germany are lucky as we have always had 3 phases into houses and offices and either 35 amp mains fuses or 55 amps.....more than powerful enough for most DIYer types of machine...a second 3 phase feed is usually available at a reasonable cost if ever needed, plus road repair.

Your comment:-

The answer is it depends on what the 100V load is.

Is a throwaway comment which as it is means at best nothing at all......but at least the grammar was good!!!

In fact a safety ground is needed for all but certain single phase double insulated equipment and is always supplied even if not always needed in all civilized countries that I know of, so really the comment is simply wrong, wrong, wrong, wrong, wrong.......

Bad hair day?

Do please remember that misleading and incorrect statements with regard to electricity can at best cause problems, at worst something really bad.....do it right or leave it alone.....

What the OP wants to do and why, I have already addressed, but as yet seen no valid answer from him....though I do feel that it is probably unnecessary in a US situation, but maybe he lives in a 230VAC country and wants to run 120VAC devices, then it might make sense, but he still needs to link one side of the secondary to make a ground/neutral, with spike for permanent installations.....and only use devices that can accept the frequency of where he lives.

Do remember that both "not reading previous posts" and "incorrect/misleading statements" will always draw attention here on CR4. Do post what you know, but post accurately and fully and don't get pissed if someone picks up on your errors and omissions.

I draw all poster's kind attention. The transformer in question is a single phase step-down one, 230 V to 110 V, fitted in old control panels, which are not reported to be malfunctioning & therefore are presumed to be working satisfactorily. You would agree that there is no need to interfere with an old well functioning system. These transformers are most likely power supply for some control circuit.

I invite OP to post the electrical schematic of the control panel to elucidate more comments from CR4 members.

I think that basically you are right and you do need to add the Neutral/earth link at the transformer if you are going to use this in your house/workshop with power tools and the like, just for safety reasons.

What you must do is to get the transformer tested to make sure that its insulation exceeds your local code before you do it. Then decide which leg is going to be earthed/neutral and add a link......

I don't know what your 220VAC input is going to come from and if you are in an area where the local code is the same as the USA, that is 230VAC 60HZ into the house, with a neutral link as a center tap, then you are simply wasting your time playing around with the transformer as you already have 120VAC....

"which do not have sencondary neutrals earthed"

There is no such thing as secondary "neutral" in the stepdown transformer, If that transformer is of the isolation type. Having said that, any of the secondary winding legs can only be arbitrarily labeled as such if you so decide and be safely connected to the earth or ground side of your circuit and as dictated by your application..

Majority of the replies in this discussion are true to their point of view. But, how will you decide that your system requires secondary earthing or not?

Many of the times, load is all electronic gadgets, that require complete isolation from the mains supply. In this case, you need not earth the secondary.

in many systems, you want to have a reference point for voltage, to protect them from leaky currents, then you may earth the secondary as well./

It depends on what code you have to follow where you live.....for full insurance cover, you need to follow it.

The older controll circuits on fans and AHU's were made with the secondary side of the controll transformers neutral not bonded to ground.

The only reason for this is that it was built prior to the NEC in US (or equal in other country) recognizing the importance of having the secondary or any neutral bonded to ground for safety reasons.

"If there is no code against it, it can be done" so the manufacturers used to build their controll circuits with isolated neutrals because they could. This served 2 purposes. A) They save money as the units were being built, less wire and less time. B) The KISS (Keep It Simple Stupid) logic used prior to recent code safety improovements allowed the concept to work very well .............

New codes recognize the safety in having neutral conductors bonded to ground (unless intentionally necessary for isolated neutral like Hospitals or sensetive electronics).

Can you bond the secondary neutral to ground ......... If its just for machine controlls ....... yes.

Most controll circuits are covered by a small glass type fuse or circuit breaker no higher than about 5 amps. If you have a "live conductor" making contact with ground in the isolated scenario - if you bond the neutral, then energize the circuit, the overcurrent protection device will pop ............ find the fault and repair it, then the circuit will function properly.

Should you bond the secondary neutral to ground .......... No.

Most technicials fail to see the floating neutral and attempt to test for power by probing the hot leg to ground, then seeing their DMM read something funky ........ like 34 volts instead of 110. The proper way to probe is to probe hot leg to neutral leg on secondary side, and then the technicial would see the 110 volts. So if bonding just for ease of maintenance, a short class on floating or isolated neutrals will cure the maintenance departments shortcomings in troubleshooting industrial electronic circuits. If bonding for sake of upgrading to recent codes, be aware of potential for OCPD to activate and test for faults prior to energizing.

In housing and offices, if neutral is not bonded to ground (somewhere), then a possible phase to ground fault will not be detected till the extra current drawn exceeds the CB or fuse rating. This may never happen as there is no proper path for current to flow through....

The frame covers of a device could then be at mains voltage.

Also ELCBs and the like cannot be used to safeguard people in such situations when earth and neutral are not bonded.

So I don't see how such things could ever be or had been code and therefore allowed....Certainly in Europe it would never ever be allowed.

America, the home of the free (to kill and maim)?

As I have mentioned before, where I live in the country in Germany, we get 3 phases 380 volts phase to phase if I remember correctly) and neutral supplied to each house.

In the house fuse box, there is a bus bar for Neutral linked to a bus bar for earth. The earth bus bar is connected by a very thick, single core cable to an earth spear (tested by the local electricity supplier for correct functionality), so neutral and ground are both driven to the same voltage level as the ground around the house itself.

Any fault current over the ground cable, diverts current from the neutral cable and ELCBs and the like should drop the power immediately.

The UK has a law requiring ELCBs and the like on all new builds and rebuilds of housing for about 40 or more years now....a great move. Should be made law everywhere....here they are legal but not yet required. I have one on each phase and more sensitive ones on the garage and garden power.

I eagerly await your reply.

"if neutral is not bonded to ground (somewhere), then a possible phase to ground fault will not be detected till the extra current drawn exceeds the CB or fuse rating."

In an Isolated transformer, since both ends of the secondary winding(s) are supposed to be floating, the question of whether to bond to earth or not is only secondary. One should be able to define first which of the secondary winding ends will be called neutral and how it is determined? And before safely bonding that so called "neutral" to earth? Please elaborate..

With a secondary that is in good condition with a high value insulation, it is unimportant as to which leg gets bonded, so long as its only one!

If you make a simple diagram of current flows and voltage differences, you will understand the problem better.....

It is a given & normal for only one of the secondary legs that should be bonded to earth If there is a need for it, at issue is which leg will best serve that purpose? No matter how high or good the insulation is, one of the secondary legs will appear to be always closer to earth or ground potential. And for best results should be tested & determined before the actual bonding to earth! A good engineering practice IMO.

Please explain to me how such testing would be made to ascertain which leg should be earthed.

Even if it were true, what would be the difference? Are you talking about a voltage or a current or a what?

Even if there was a difference, what difference would it make if the other leg was grounded?

Please answer the questions......I am fascinated by this line of reasoning about this......but cannot follow the reasons or the methods without some facts.

Andy, It is an easy to follow test procedures /or to understand the logic behind even for none EEs.

1. To determine which leg of the secondary is best bonded in an isolation type stepdown transformer. For a 220/110V transformer, simply apply the 220V input to the primary, then using a VOM, verify the expected 110V output on the secondary side. Then select a good power earth ground and use that as the voltage reference test point for the following; verify that each leg of the 220V input, (In the U.S., it will be between L1 and ground, as well as between L2 and ground), on a balanced power distribution, it is expected to equally register a 110V readings or very close, for each leg on this steps. Now using that same earth ground as the common test reference point of the Voltmeter, take voltage measurements from each of the secondary leads. Compare and observed both readings, one of the voltage readings is or will be lower than the other, but when summed together they should equal the total 110V taken initially at the beginning of this test. Now whichever leg that reflected the lower reading, indicates that it is much closer to the ground plane and therefore should be the one bonded to the power ground, if needed.

Applying the above testing practices for transformers in industry applications, be it in audio amps, power control systems, or instrumentation, most of the unexpected nuisances such as unwanted noise, ground loops, circulating and/or leakage currents were greatly minimized if not completely eliminated.

As we were talking about the secondary, why are you now talking about the primary? What has that to do with it?

You wrote:-

verify that each leg of the 220V input, (In the U.S., it will be between L1 and ground, as well as between L2 and ground), on a balanced power distribution, it is expected to equally register a 110V readings or very close, for each leg on this steps. Now using that same earth ground as the common test reference point of the Voltmeter, take voltage measurements from each of the secondary leads. Compare and observed both readings, one of the voltage readings is or will be lower than the other, but when summed together they should equal the total 110V taken initially at the beginning of this test.

You now test the L1 to neutral and the L2 to neutral. Do remember that the voltage on each will be a result of loading, probably unbalanced, so the two voltages may not be equal due to that alone. Also they may not even add up to 220VAC (not 110 as you claim) either. What does this prove????? Answer:- NOTHING AT ALL.

Testing between the primary side neutral and the secondary side will show probably a few micro volts of "noise" only, probably nothing as in a properly insulated transformer, there are no paths between the primary and the secondary......certainly nothing like 110Volts as you claim. The noise measured will vary according to the test equipment, an Oscilloscope showing the highest voltage, a DVM next and an old fashioned meter the least....

You appear to have forgotten that the transformer primary will be to both L1 and L2 and the neutral will not be used.....so any voltages relative to neutral do not play a role anyway when you connect a 220 to 110 volt Transformer..........

You then make a big failure with the comment:-

Now whichever leg that reflected the lower reading, indicates that it is much closer to the ground plane and therefore should be the one bonded to the power ground, if needed.

I am of the opinion that any transformer, with in excess of 1 meg ohm insulation (most will be significantly higher) between secondary and frame will show the same identical tiny reading at either end of the secondary.Due simply to the fact that the insulation between primary and secondary will usually be in the order of either winding to frame x 2. Anything less could indicate a possible problem. We are assuming that the transformer has been checked with a megger to demonstrate good insulation.

So in fact, it makes no difference whatsoever which output leg gets earthed.

To be blunt, you demonstrate a singular lack of understanding of electrics that I have only seen and heard equaled by certain semi skilled electricians....or very recently qualified engineers... you need to learn more before you kill yourself or someone else.....

To sum up, your test procedure is a) wrong and b) completely unnecessary.

I remember such an RN instructor, who hadn't a clue, I showed him an incomplete ring main, I told him I was going to complete the ring, he said it would explode. So I CONNECTED IT BEFORE HIS EYES AND HE RAN AWAY IN FEAR. Of course it did not explode.....it worked as designed and with less voltage drop as any ring main is designed to do....ask anyone with a house in the UK for example !!!!

A little knowledge, especially if inaccurate, is highly dangerous......you have demonstrated that well today.....sadly.

Your starting comment was:-

Andy, It is an easy to follow test procedures /or to understand the logic behind even for none EEs.

I agree fully, it is easy, but not for you it seems....

By the way, you should have written "non EEs", not what you wrote!!!!

A good thing too!!!

You finally giving up I mean, but we have enjoyed the Fairy tales......makes a change here on CR4....

Please don't. Save yourself some grief. If you want to know whys' then please post schematics.

Who were you replying to?

I goofed up in that the reply was intended for the OP. Sorry about that.

OP said:"The company that I currently work for has some old control panels that have step down transformers (230to100vac) which do not have secondary neutrals earthed." OP does not report any problem with the working of the panels. OP further says, "I am tempted to attach an earth to the secondary neutral, is there any reason why I shouldn't???" I say, is there any valid reason for him to indulge in his temptations, specially since there is no report of any malfunction? Now, tell me Andy, how do you feel?

PS: I presume, these transformers, in the control panel are likely to be, very small power supply units, supplying power to relays, solenoids etc. etc.

Let sleeping dogs lie!

He has not given us enough information to ponder further than I have already....

Even a tiny transformer with 120 VAC on it can kill an unwary user......

The equipment frame is always protected by an earth. So the question of "Even a tiny transformer with 120 VAC on it can kill an unwary user......" does not arise. One that puts his finger inside, surely knows, what he is doing, otherwise he has no business to be around in the first place. Even your proposed grounding of the secondary can't help him either! Now about, "He has not given us enough information to ponder further than I have already...." If you want more info. then ask the OP. It is better not to continue with the thread...if the OP hibernates.

Even if one touches only one end of the secondary, he will not be electrocuted. The secondary out put is not with respect to grid neutral or earth. Then there are safety fuses to take care of leaks & short circuits in the transformer. How will you ground secondary of the transformer, where the secondary is connected to a bridge rectifier? One should not ponder, but get facts first from the OP. Lastly, these are my considered views & I shall defend it.