Newton Tesla lathe package

I may have made an assumption that the NT box EStop will just stop the motor but the additional EStop would stop the CNC as well. In the event of a crisis with the CNC you will have to remember to hit the extra EStop as the NT button will not stop the CNC. As it is likely you will be the only user in your own workshop then the functionality of the 2 buttons will be known but in an industrial situation you would expect hitting the nearest EStop to stop everything.

A VFD normally behaves as though it has an NVR in it so in the event of a power failure or hitting the EStop it will not restart when power is restored, some VFDs have a parameter that will restart the motor when power is restored but this is usually used where no danger will arise from an automatic restart, applications like a pump in a remote unmanned location where it would be desirable to have the pump restart as soon as power is restored. Obviously restarting a manually operated machine unexpectedly could be dangerous.

Mike

There are several different ways of providing emergency stop depending on the nature of the emergency.

If there is a mechanical/operator entanglement emergency the machine may need to be stopped as quickly as possible which may require a mechanical brake or an inverter to apply regenerative braking. Having an eStop that removes power from the inverter prevents regenerative braking.

However if there is an electrical emergency such as fire or electrocution, then an eStop that removes all power, is appropriate.

An inverter eStop will trip the inverter and will require power to be cycled to reset it or a dedicated input used as a reset.

In a complex machine a risk assessment and FMEA will govern what/who handles an emergency stop and how it is handled.

Modern electrical safety requirements requires any device made of a conductive material (metal) to be earthed whether there is electrical power connected to the device or not. The thinking here is that the device could become live if a device with an undetected fault came into contact with it. So for instance a steel central heating radiator in your house is required to be earthed. This is normally provided by the copper pipes but with the increasing use of plastic pipe and fittings earth continuity may be lost and supplementary earthing is needed.

An electrical machine such as a lathe would normally have two independent earth connections with one that can be visually checked for presence. The thinking here is that a single failure, ie broken earth wire, should not cause a machine to become unsafe and no special equipment is needed.

Domestic dwellings now have to have a residual current trip device (earth leakage trip) but there are many that have not been upgraded so good earthing practice is always wise and you can't have too many belts and braces.

Clive S

Bump

Clive, I am retired now but have never recalled a regulation calling for 2 independent earth conductors being required to the frame of any machine in a normal home or industrial installation.
Is this a recent requirement ?

I do remember that when testing fixed equipment the maximum cpc resistance had to be below a certain level to ensure any fault current could flow to earth so rupturing the supply fuse or tripping an mcb.
Such tests were carried out by injecting a 25A current into the earth lead to measure the impedance.

Emgee

Surely with multiple earth conductors to one equipment you can end up with parallel earths that in certain circumstances can build up a lethal potential, I seem to recall some deaths being caused from this type of situation when I was in the Air Force, this was on power sets supplying power to aircraft electrical systems. I witnessed hefty blue sparks leaping to the airframe from independently earthed equipment being used on aircraft connected to ground power units, “wiggly amps” not my trade so I learned to treat this condition with respect. Dave W

All a very interesting discussion but could anyone run a rule and possibly their phone camera over how the unit mounts to the Myford stand please?

My unit is 210 wide x 280 high x 90 deep (front to back) and has c 10 wide flanges top and bottom for mounting.

That's brilliant ega, many thanks. One other thing - what is the mounting hole spacing please? And are there holes at top and bottom flanges?

Thanks, John.

The requirement for two earth usually applies for industrial equipment and a lathe, although being used in a domestic environment, is still industrial. A usual the requirements are written in such a way that they do not necessarily describe the solution just the needed requirement. So the requirement may specify the need for a "fault disconnection device" known to me and others as a fuse. If you look on the back of some equipment you'll see the the normal IEC 3 pin connector for the mains power and close by a threaded post with a earth symbol next to it. This is for the second earth wire. A lot of domestic devices don't need a second earth because there are no exposed conductive parts that can be touched by a "Standard finger".

The second earth on a industrial machine may be via the metal conduit the supply comes through and if there is a flexible section there should be a seperate earth wire wrapped around the flex tube in case it gets stretched and becomes disconnected.

The normal PAT test does not cover the electrical tests needed to prove product compliance.

Check out IEC 61010 and IEC 60204-1 if you can't get to sleep at night.

Clive S

To cover another point that was made regarding earth wires. Earth wires shouldn't carry anything other than fault currents and then only for the time needed for supply disconnection. If it is then it is a supply connection intended or otherwise. There are complications where a supply wire is bonded to an earth wire eg neutral to earth and early electrical standards or lack of standards caused dangerous situations. This could arise if one bit of kit was connected to another bit of kit made to a different standard. All wires, just like any aircraft propeller, should be treated as live until proven otherwise and electrocution isn't an appropriate method of testing.

Clive S

There are two c 5mm holes in each flange symmetrically placed at 160mm centres.

Incidentally, my unit is on the right of the lathe stand and near to the mill which it also serves, an arrangement agreed with NT, saving money and allowing a tolerably rapid change over.

That's perfect! Many thanks again.

Clive

The second earth you refer to in a conduit supplying power to a machine is a supplementary earth to ensure the impedance of the earth return is a low enough value.
This additional conductor may not be required to have a low enough impedance value but is considered good practise in such an installation.

PAT is not designed to prove product compliance, it is to determine if the item is electrically safe to use.

The IEC papers you quote have nothing applicable regarding earthing.

Emgee

I think I did say that the PAT test does NOT cover the tests needed to prove product electrical safety compliance.

It's been some time (13 years) since I last had to read and design products to comply with the Machinery and EMC directives but as far as I recollect IEC 61010 was to do with product electrical safety. The specifications may not mention earthing per se as this is only one way of achieving electrical safety. A double insulated device doesn't need an earth and a lathe may be classed as such if the motor and associated electrical system are installed in such a way that a failure of insulation or clearance will not result in an accessible conductive part becoming live. However when the lathe is installed in a factory or workshop another assessment is needed to ensure the product remains safe to use and earthing may be needed and two earths one of which can be visually inspected helps maintain safety compliance which may be defined by the Factory Act or other electrical safety requirements.

Clive S

A quick check of IEC60204 shows there is a section on "Protective Bonding requirements, terminology and protection against electric shock" commonly called earthing by laymen.

Clive S

Clive

Read your first post to see what you said regarding PAT.

Perhaps you can post copy of IEC 60204-1 5.3 and clause 6 to enlighten me further, particularly the section that states 2 cpc conductors are required.

IEC 61010 I found refers to lab measuring equipment, not machinery installations.

I don't believe the specs require two earths but if the electrical safety of a machine or instruments relies on a earth being present then a single failure of that earth connection could result in the machine or instrument becoming unsafe. One way to overcome this situation is to have two earths so if one failed there is the other. However if both earth wires are hidden then you may not know that one has already failed and you're back to single failure situation. Regular testing or a method to prove the earth is present before power can be applied is possible but a visual check is relatively simple to do.

Every a domestic ring main socket outlet has two earth wires and an earth wire from the socket earth to the metal back box in case the screws don't provide good contact or get left out. Another example is a domestic metal kitchen sink is required to have second earth in case one isn't provided by the pipework.

Clive S

I think you will find that is called suplimentary bonding.

In my 50 some years as a sparky we never ran 2 earth wires . Sometimes the conduit was an earth & the installation would have an earth wire too. But a plugged in appliance with a 13a plug top fitted would never have a extra earth wire. Where would you connect it. I think were some are not understanding this is.

On my Myford lathe a young 1970's model. It had a cable with a 13a plug fitted . In the motor control box where the earth connected , there was another earth wire attached , which came out of the terminal box & bolted to the side of the lathe bed. Not sure if that was standard or done by someone over the years.

It would not hurt to take a earth from your incoming cable connection & run to the frame of any machine.

Steve.

It's unfortunate that a small aside in what was mainly a request for information has been derailed into an interesting but irrelevant discussion on electrical earthing! To be clear, I intended no criticism whatever of the NT system, I just commented on a Myford quality problem. Maybe people who want to carry on debating the safety topic could start a new thread?

John

Apologies for leading your post to a different topic but as you pointed out machine frame earthing is an important issue.

I have no intention to post further on the subject.

Emgee