88V to earth from secondary of 12V 10A power supply....

[bowlingo]

Hi all,

I had a 10A 12V power supply today of which is switching 2 x 2A 12V pumps wired in paralel. I had a relay switching the primary side of the transformer...This was working fine until yesterday when I put my hand in the tank and got an electric shock!

I was then measuring 30 - 100V between + or - of the 12V secondary side (+ and - was giving 12.7V) to the concrete floor of the garage I had the pump running in. When I measured to earth on the back of a socket between the + or - I was getting 88 volts.

The meter was saying the polarity was correct if I connected the black lead to + or - of the secondary 12V side and the red leas to earth..if I connected it the other way round it was saying reverse polarity.

I then went off and bought another 12V 10A power supply and connected it as before then after a while the 8A relay switching the primary side stuck on (burnt out)

Should I be switching the primary or the secondary of the transformer? I think switching the primary caused the transfomer to burn out but I have no idea why it would give 88V to earth from the secondary of a 12V transformer of which was giving a steady 12.7V between + and -

Anyone any ideas please?

Thanks

 

[GreenGiant]

You had it set to AC or DC and it was reading 88V?

If AC that sounds like a bad ground somewhere, and you are getting leakage from somewhere in the device where it is supposed to regulate it to +DC (Im assuming the 12V is DC)

Have you checked ground to another ground and see what that reads?

 

[bowlingo]

You had it set to AC or DC and it was reading 88V?

If AC that sounds like a bad ground somewhere, and you are getting leakage from somewhere in the device where it is supposed to regulate it to +DC (Im assuming the 12V is DC)

Have you checked ground to another ground and see what that reads?

It was set to DC...

Do you mean do an earth continuity test between the earth of the socket and the main earth terminal of the consumer unit?

I added an earth to the secondary - also by the way as when it happened it bought the voltage down to 0.2 - 2V

 

[GreenGiant]

It was set to DC...

Do you mean do an earth continuity test between the earth of the socket and the main earth terminal of the consumer unit?

I added an earth to the secondary - also by the way as when it happened it bought the voltage down to 0.2 - 2V

Yes earth continuity

if it was reading 88VDC that sounds like a regulation issue, Im not experienced with those systems but somewhere where it regulates the 120/240 from the mains to 12VDC there is something amiss

 

[shrtrnd]

Floating ground alright.
I had a metal desk a long time ago with a composite top. I screwed one of those long-armed flourescent table lights into it so I could see what I was doing. I had one of
the old rotary dial phones on the desk. Everytime I dialed the phone and touched the
metal finger stop on the dial, I got a shock.
Took me a month to figure out the flourescent lamp housing I had screwed into the metal
desk, was at a different ground potential than the telephone line ground.
There was a difference in potential (voltage). When I leaned against the desk (lamp ground), and
dialed the phone, telephone company ground, I got zapped.
Funny now, but it took me a while to figure it out.

 

[bowlingo]

can anyone help me with how to find out exactly what earth potential the sockets within the garage earths are at?

Today I did a Zs test and was getting 0.42 ohms of which is good

I also did an RCD test and it was disconnecting the main switch RCD within the consumer unit within the house at normal times...around 25ms at 1 x 30mA rated test and around 15ms at 5 x rated test.

As I understand it if the RCD goes within the house and I have a good Zs it should all be at the same potential?

I even plugged in an extension lead from a socket within the house and ran it out to the garage and was still getting the 88V from the secondary side to the sockets earth...

In effect what I am doing is relying on the earth wire that ive connected to the pumps negative...if this breaks or comes off then I could be in trouble

 

[duke37]

In the UK we have PME (phase, multiple earth?) where the neutral is connected to local earth and everything is bonded together in one premises. This is not 100% reliable.

1. A grain dryer developed a fault, line to local good earth, and this pulled the neutral up to 100V or so. Thus other consumers had a high earth. The electric supplier put in extra earth at the transformer.

2. A distribution transformer developed a fault and lifted a steel framed building to 80V. This was confirmed by measuring the voltage between local earth and a peg in the ground 20 yards away. A sensitive detector was a sheep who objected to a shearing cutter at 80V. The 11kV transformer was replaced.

I do not understand why your relay should stick unless you have excessive current through it. Are you suppying the earth for the neighbourhood?

 

[bowlingo]

In the UK we have PME (phase, multiple earth?) where the neutral is connected to local earth and everything is bonded together in one premises. This is not 100% reliable.

1. A grain dryer developed a fault, line to local good earth, and this pulled the neutral up to 100V or so. Thus other consumers had a high earth. The electric supplier put in extra earth at the transformer.

2. A distribution transformer developed a fault and lifted a steel framed building to 80V. This was confirmed by measuring the voltage between local earth and a peg in the ground 20 yards away. A sensitive detector was a sheep who objected to a shearing cutter at 80V. The 11kV transformer was replaced.

I do not understand why your relay should stick unless you have excessive current through it. Are you suppying the earth for the neighbourhood?

I have taken the old power supply.....

http://www.maplin.co.uk/12vdc-10a-power-supply-with-2.1mm-tip-513521

home with me (different supply) and am still getting 88V DC between the + or - to the earth on the back of a socket and its 12.7V DC between the + and - of the supply

Could I of damaged this by switching the primary on and off with a relay?

 

[rbindl]

Your grounding system is bad. Assuming you have an earth ground established, the output (12V) side should be earth grounded since you are dealing with water and people can stick their hand in it and get a shock. The power switch appears to be bad since you have replaces the power supply. How is the switch tied into your circuit? Any common mounting points?

 

[duke37]

When you say 'different supply' do you mean a different power supply or different source.

Check the resistance between the mains input terminals and the output terminals, it should be open circuit.

A switch mode supply may not like being turned on and off often as it has a large input capacitor which needs to be charged up each time. A relay is no different to any other switch but needs to be strong to withstand the surge.

There may be some capacitance between input and output which can give an indication on a sensitive meter even if the current is very low. I suggest that you power the supply through a 30mA trip and connect one of the output terminals to an earth and see if the trip drops out.

 

[bowlingo]

When you say 'different supply' do you mean a different power supply or different source.

Check the resistance between the mains input terminals and the output terminals, it should be open circuit.

A switch mode supply may not like being turned on and off often as it has a large input capacitor which needs to be charged up each time. A relay is no different to any other switch but needs to be strong to withstand the surge.

There may be some capacitance between input and output which can give an indication on a sensitive meter even if the current is very low. I suggest that you power the supply through a 30mA trip and connect one of the output terminals to an earth and see if the trip drops out.

Different 230V AC mains supply from a different house (my own house)

 

[john monks]

Switching the primary of a transformer with any kind of voltage suppression system, like varistors, can cause voltage spikes into the thousands of volts and short out a transformer. Yes maybe the relay switch should be on the output side. But if you are not familiar with line voltages around water I suggest you pick another project. Too dangerous.

 

[shrtrnd]

I don't kinow if you're in UK or Australia, you talk about 'mains' which we don't do in
the States.
Our power comes off the outside transformer at 3-Phase 480VAC.
In larger buildings, or distant buildings on the property, when the electricians wire the
buildings, they try to 'balance' the 3-phase load, by picking-off 3-each 120VAC circuits
to evenly distribute the load. If one of the single phase loads is compared to any of
the other single phase loads, there is a difference in potential, as the electrical neutral
of each phase is at a different potential. I've found damage to equipment in some buildings, because
people will plug one piece of equipment into one phase, and then plug something else
into another phase, and then combine the equipment to perform a function.
It's called 'cross-phasing', if you want to look it up.
In this particular case, I think the info you got from most people here is correct:
Your ground is not right.
You house may be grounded correctly.
Your garage may be grounded correctly.
But they're not BOTH grounded to the same point.

 

[bowlingo]

I have taken the old power supply home with me and plugged it into a socket (UK 230V AC..my house voltage is in fact 241V) and between the + and - of the secondary of the power supply I am getting 12.7V which is fine. When I then connect the black lead of my multimeter between the + or the - of the secondary of the transformer and put the red lead against the earth on the back of the socket I am getting 88V DC...This is the same as I was getting at the garage of which is a different mains supply and 4 miles away.

 

[shrtrnd]

Alright, I know it's a pain, but we consider what we think we hear about your problem;
and that's not always what you're seeing.
1)What's the make and model of the meter you're using to take this 88V measurement?

 

[bowlingo]

Alright, I know it's a pain, but we consider what we think we hear about your problem;
and that's not always what you're seeing.
1)What's the make and model of the meter you're using to take this 88V measurement?

Cheap Maplin multimeters x 2 and a calibrated Meggar all in one tester

http://isswww.co.uk/17th-Edition-Te...52-Multifunction-Tester-with-3-year-warranty/

I spoke to a friend up north earlier who has owned a small electronics company for many years...I am going to post it to him and see what he can find out

 

[gorgon]

I have taken the old power supply home with me and plugged it into a socket (UK 230V AC..my house voltage is in fact 241V) and between the + and - of the secondary of the power supply I am getting 12.7V which is fine. When I then connect the black lead of my multimeter between the + or the - of the secondary of the transformer and put the red lead against the earth on the back of the socket I am getting 88V DC...This is the same as I was getting at the garage of which is a different mains supply and 4 miles away.

If this is a switchmode supply, it could be that you may have a static voltage buildup between the primary earth, and the floating 12V. I've seen similar buildups of 150V on isolated laptop switching powers, making RS232 ports migrating to hell.

The real question is then, is there any current flowing here?

Take a normal lightbulb and connect it between mains earth, and -12v. Measure the voltage over the bulb, both DC and AC. If it is zero, you can verify the zero current draw with an amp meter.

If there is no current flowing here, there is no real problem. If there is a current here, your power supply is not according to normal standards, and it looks like a leakage to earth.

TOK

 

[bowlingo]

I posted the faulty PSU to a friend up North and he has just e-mailed me with the below...

That power supply was an odd one... there was no earth connection. Most of that type (laptop PSU) I could find about the place did have the output connected to earth. I do know that is is not the case on more and more power supplies. The problem with that unit of yours was that the output was not isolated from the input there is some capacitor link between input and output. I connected an ampmeter between ov on the output and earth and measured a constant 2mA AC. Although not lethal it will bite you as you well know. The other problem is ... it is more than enough to kill some electronics that you may have connected to a laptop ... so be careful using such PSU's. Maybe connect the output to earth if it is like that one....

 

[shrtrnd]

Floating ground.
Your friend is right, that's hazardous to your circuitry.
If there's a way to safely ground both units to the same point, you can try it.
Otherwise, you may zap your gear one day using the power supply 'as-is'.
I know it's not what you want to hear, but you need to protect your electronics.