Newbie- sensor for ground(machine body)

[dodod3d3]

Does any body here have schematic or diagram, sensor for ground while cable ground cut off or not connected.
I will use this sensor to interrupted the machine when cable/wire to ground has suddenly disconnect or broken.
Thanks for your help in advance.
Regards.

[Al_The_Man]

Are you intent on building one? or just buy one.
Depending where you are they are called ground fault interrupters or Earth Leakage Trips.
For a large machine it may cost as you have to monitor all feeds to the machine and detect a current imbalance between conductors, indicating a ground fault has happened.
If you just want to detect the validity of the ground conductor, then you would need a method of constantly passing a small ground current and then detect the loss.
Do you anticipate losing the ground conductor?
Al.

[dodod3d3]

Thanks Al for your respond.

I intent to make a simple circuit to monitor the ground fault (with 24 dc )because i need to use this signal to the plc to interrupt the machine..

the idea is just like in wire cut machine,the sensor detect if the wire contact the workpiece or if wire has broke in the middle of process.

[Al_The_Man]

What you could do is use a opto with isolated supply to the input LED, 4N35 etc.
If you use a 12v or 24v to supply the bias current it should be isolated from any other connection.
Connect pin 2, LED cathode to your ground star point that you need to monitor, this must be connected to a good earth ground.
connect LED pin 1 anode to a resistor to limit the current to around 30ma connect the other end of the resistor to your isolated supply -ve the other end of your isolated supply to a separate earth ground rod used expressly for this purpose.
The output of the 4N35 can be used sink or source into your PLC depending on the nature of the inputs.
This will however not only monitor your machine ground, but the validity of your ground rod set up.
A high resistance on either will cause the alarm.
Place a .1amp fuse in the isolated supply.
Al.

[wildwestpat]

Hi Dodo3D3

Q. Does any body here have schematic or diagram, sensor for ground while cable ground cut off or not connected.

Response I assume you want to operate a switch that will halt the machine but not disconnect it from the mains supply so that the job can be resumed when the fault has been sorted.

I must point out the risks in that any earth continuity fault has the potential (no pun intended) to cause harm if not death so please take care of both yourself and any 'visitors'.

Al has indicated the use of earth leakage trips that provide safety by limiting the current that can escape to earth. For safety reasons these are often mandated by legislation in Europe for workshop use and are normally part of the electrical supply and limit both the fault current as well as the earth leakage. Best advice here is to have these in use and tested every now and again. They have a test button that will bleed a bit more than the limit current to earth so they test both the supply earth as at the distribution board as well as the trip.

However the earthing at the machine could be tested in one of several ways.

1.provide a simple test button with a limiting resistor between the machine earth / frame and the live side of the supply. Operating the button which must be insulated and in an insulated enclosure will then trip out the distribution board trip.

2.Use a current transformer to detect the out of balance supply and use a secondary winding to detect that imbalance to operate your emergency stop.

3.Use a simple isolated supply to send a current between a known good earth and the machine frame and use this to energise a relay. The current could be ac or dc. The value of voltage and current can be adjusted to suit the components available but the applied voltage should be kept low in the interests of safety and the current limited by the relay coil and a suitable padding resistor. An LED could be placed across the resistor to indicate earth OK. This is the method used to test a new electrical installation. An earth spike driven into the soil out side the premises taking care to avoid any buried pipes and wires would provide a suitable terminal. Note the use of an earth spike is used as part of the supply testing in Europe (UK for certain) on new installs.

PLEASE TAKE EXTREEM CARE AS THE EARTH BONDING AND SAFETY WILL BE COVERED BY LEGISLATION AND IF IN DOUBT A QUALIFIED ELECTRICIANS HELP SHOULD BE SORT.

If you are close to the distribution transformer of the electricity supply company then there may be a varying voltage between true earth of the earth spike and the company cable earth.

Hope this helps and above all stay safe

Regards

Pat

[wildwestpat]

Hi

Just read Al's post. Strongly suggest you use an ac test current to avoid any long term corrosion problems arround the earth spike caused by passing a current only one way. I also think that a current higher than thiry miliamps would be advisable as there will be public supply currents flowing in the earth / soil and these are variable my gut instinct is for a current of 100 to 300 milliamps which would be sufficient to operate a relay coil via a small bridge rectifier. Looks as if you are now on track but do make sure all your equipment is properly connected to the supply earth.

Regards

Pat

[dodod3d3]

Many Thank To All & Pat...

To All>>I will try your idea...look for the circuit yet

To Pat>>Yes this circuit just for interrupted the process only not to off the mains supply.

i prefer to use 24 v dc because is already there.

Anyway thank for your ideas.

Regards

[wildwestpat]

Hi Dodod3d3

There will be a problem with corrosion arround your earth spike due to the current flow if you must use DC. The current will set up a simple plating cell hence the corrosion could be quite rapid. (Might be worth fitting a reversing switch so that alternate sesions have the +&- terminals reversed from the supply. Use two LEDS back to back i.e. in parallel anode to cathode if you want to keep the wiring simple.) However since you are not relying on the earth spike for the safety earth you will have a fail safe system as the earth connection when it fails will trigger your alarm system. You will also need to keep the earth spike used for monitoring well watered if the soil is not damp. Electrical supply outlets have neat plastic inspection pits for use with earth spikes the cost in the Uk is arround £30 including the 1meter spike.

Regards

Pat

[Al_The_Man]

I would not think that 30~100 ma would cause any corrosion problem?
On a side note, I recall that in the UK at one time before the general use of earth leakage trips, when Earth ground conductor was allowed as a reference to the supply transformer grounded neutral, an installation had to conform to a min resistance, (low ohms) and was measured using the neutral conductor and the ground conductor, here the actual ground resistance was measured back to the transformer.
Even if the service supply carried a ground conductor, it was not allowed to use it.
dodod3d did not mention where he is located so it is unknown what type of ground systems exits at the moment.
Al.

[wildwestpat]

Hi Dodod and Al

Having just read through this thread again I am unhappy about the concept. The provision of electricity in a safe way is a real headache and has resulted in tons of legislation and Health and Safety rules. To answer your question since I have already commented too much to pretend I did not know the risks I will set out the risks and difficulties.

1. The exposed metal work that can normally be touched with a small finger needs to be at earth potential if supplied with electricity from the public supply. Unless it is constructed in such away that all electrical parts are double insulated. Most electric hand tools are double insulated and are appropriately marked and fitted with two core cable. Most machine tools such as mills and lathes are not double insulated and must have their metal work earth bonded. This usually interpreted to mean that the metal work of the machine should have a bonding wire that bonds the metal back to the incoming mains earth separately from the power cord earth. (In most cases this should have a cross sectional area 3 times that of the supply conductor.)

2. The use of an earth leakage trip in the mains supply is recommended as this will help reduce the risk of a fatal electric shock. Two sensitivities are commercially available - use the lower current one unless advised otherwise. In the UK this would be a 30ma trip current - still capable of giving a nasty shock but one that should be survivable.

3. The earth potential of the supply companies earth as seen at their meter and the potential of the ground on which the workshop stands should be low but there are conditions that may give rise to either a continuous potential difference or a varying one. This is dependent upon the local supply relationship with the supply substation and the local geology.

4. It is good practice and is probably mandated that all incoming services are bonded together. Think water - gas - central heating - bathroom etc. This is to address the problems caused by plastic water and gas pipes as well as non metallic pipe fittings.

5. The voltage that exists between live and neutral supplies the 'pressure' to operate your desired equipment. The voltage between either with respect to the supply earth can vary and it is not unusual for the neutral to be tens of volts with respect to ground. In some countries there is signalling over the cable as well as the supply of energy.

6. The proposal to detect an increase in the resistance between the exposed metal of the machine and the power company earth posses a conflict to interests. Having knowingly detected that the machine is no longer earthed there is probably a duty of care that means disconnection of the supply and rectification of the fault. One could speculate on how the coroner would interpret the desire to continue machining knowing that the machine earth was suspect.

At this point I have gone from being enthusiastic to backtracking due to the safety implications of what you intend to do having been made aware of the potential fault.

Hope this helps rather than just salve my professional conscious.

Kind regards

Pat

[Al_The_Man]

One problem here is we do not know what jurisdiction that the O.P. is working in.
I am also not sure what his exact concerns are as to the validity of his machine ground path.
The differences between N.A. and UK for example are different, and I am aware that changes have occurred in the UK since I was involved in the electrical industry there, especially as to grounding of the service, for example, the current practice here in N.A. on bonding and grounding the neutral, together with the service supplied ground conductor at the customer service panel was a definite no-no in the UK from my past experiences, although at that time ground fault interrupters were not common and ground conductors were allowed provided the ground resistance from installation to supply transformer conformed to the required code specification.
I am assuming that if a ground failure was detected, his intention is to cause a disconnection of the machine?
Normally if the local electrical codes have been conformed to there should be little concern in any event, in most jurisdictions anyway.
Al.