PDA

View Full Version : Grounding and probe



ryghar
16-04-2019, 06:00 PM
Hello!

Reading about proper grounding of cnc electronics I saw that ground has to be isolated from power supplies negative. So, I have a copper plate with ground cables from 48v, 12v and 5v power supplies with ground cable from the wall.
Everything works fine but the machine itself, has no ground cable because if I attach the negative of my probe cable I will be connecting negative to ground... This morning touching the machine structure and the door handle (the door is metalic) I felt a light discharge. Should I connect the maching to ground? Can this affect in any way the electronics with that negative to ground? ( the discharge was lightly, I can live with that lol)
The router has no ground cable, maybe that was the reason for the discharge.

Thanks!!

john swift
18-04-2019, 01:30 PM
double insulated equipment with no earth / ground connection can help avoid problems with ground loops when multiple pieces of equipment are interconnected
but can be present a shock hazard when they use switch mode power supplies
( the leakage current from one supply may be safe but several supplies can add up and be a shock hazard )

I would connect an earth connection to the cncrouter frame
more detail of your system is needed
what are you using as the spindle ?

you may get away with the static discharge but if it finds its way into the control electronics via the limit switches for example it can damage sensitive electronic devices

John

m_c
21-04-2019, 12:44 PM
Any kind of metal frame should be connected to earth. It's a basic safety requirement for industrial machines.
John mentions double insulated equipment, but you would really struggle to create a CNC router that could meet double insulated requirements.


However, grounding and earthing can be a bit of a complex issue, but the basic issue is you want to avoid ground loops. That is where the same wire is connected to ground at both ends. An example might be a bit CY signal cable where both ends of the screen have been connected to ground, which can result in current flowing through the screen causing interference in the internal wiring instead of protecting the internal wiring from interference.


My usual wiring practise is to have a central star ground/earth point within the control box (I personally use earth DIN terminals that connect wiring to the DIN rail, but a single stud to the case will also work), which is connected to earth. I then connect all the PSU 0V to the same terminals, so all supplies are connected to earth. I then ensure any shields are connected at only one end (either directly to the case if terminating using glands at the case, or a drain wire to the nearest gnd point - there is no need to run all screen drain wires back to the star point).
Also, if you want to really minimise interference problems, spend a bit extra on a BOB that uses 12 or 24V control wiring.

cropwell
21-04-2019, 05:45 PM
Some people think that single phase AC Neutral is synonymous with Ground (Earth). It is not! It is phase neutral for the live, which is one of the three phases used for domestic supply. Different houses in the same street can be supplied from different phases. That, however, is a digression. There is often a voltage difference between Neutral and a local earth. That voltage will depend on several factors. At my house it is 10v. Power supplies can be double insulated, but the output Ground should be connected to Earth, as should the whole machine. In some countries, the Earth terminal on the Mains socket can not be relied on to give protection from shock hazards and so a ground rod is desirable.

Clive S
21-04-2019, 11:17 PM
Some people think that single phase AC Neutral is synonymous with Ground (Earth). It is not! It is phase neutral for the live, which is one of the three phases used for domestic supply. Different houses in the same street can be supplied from different phases. That, however, is a digression. There is often a voltage difference between Neutral and a local earth. That voltage will depend on several factors. At my house it is 10v. Power supplies can be double insulated, but the output Ground should be connected to Earth, as should the whole machine. In some countries, the Earth terminal on the Mains socket can not be relied on to give protection from shock hazards and so a ground rod is desirable.

Yes Rob but generally in the UK in towns not necessary in the country The neutral in generally connect to the earth at the consumer unit. Just saying:thumsup:

cropwell
22-04-2019, 12:06 AM
Yes Rob but generally in the UK in towns not necessary in the country The neutral in generally connect to the earth at the consumer unit. Just saying:thumsup:

In my house (in the country), the neutral is definitely not connected to Earth as if I short N to E it will trip the RCCB, by causing a Live-Neutral current imbalance of > 30mA. The wiring was brought up to regs about 5 years ago with a new consumer unit.

The OP lives in Buenos Aires* and I do not know the regs in Argentina or the state of his workshop wiring.

*If his flag header info is correct.

Rob-T (just to avoid confusion)

john swift
22-04-2019, 06:37 PM
For one of the supply wires to be "Neutral" it has to be connected to earth / ground other wise it would be just another line

the earth connection has to be a low enough impedance to ensure the fuse or circuit breaker opens fast enough to remove the danger when you have live to neutral or live to earth fault

Here in the UK
Depending on the wiring system used in towns and cities the utility company may supply the earth and neutral as separate earth and neutral conductors (TN-S system)
or as a combined earth & neutral (TN-C system) some times with multiple earths along its length

For rural supplies the you may have to supply your own earth (TT system)
even though the utility will have connected the neutral to earth at the transformer

It does not matter if the neutral / earth connection is 3 miles down the road at the supply transformer or the utility junction block next to your Kw/Hr meter

what matters is the neutral is earthed before it passes through the residual current circuit breaker ( RCCB)
what you see when measuring the difference between Earth and Neutral is the volt drop along the neutral wire when the 3 phase supply cable has an unbalanced load

when ever the neutral wire out of the RCCB is connected to the earth there is an imbalance between the live and neutral currents which trips the breaker - (you have an earth path in parallel with the neutral reducing the neutral current the RCCB sees)
usually the circuit breaker is designed to trip at 30mA
( depending on the application various trip currents are available )

In my part of South Lancashire my mains supply varies between 243V and 248V
depending on the time of day - just now its 245V

John

cropwell
22-04-2019, 07:11 PM
If Neutral is connected to Earth, there should be no current flowing to cause a Live-Neutral imbalance. However, when I accidentally shorted a light fitting neutral to earth With the light switched off, the whole house was sent into darkness. I have a very good local earth strapped to a copper water feed pipe. This leads me to assume that, although the phase neutral may be earthed at the substation, there is enough of an induced voltage in the supply line on its' journey to my house to cause a RCCB trip if earth is connected to neutral.

My point is that, UK has regulations in place to try to ensure electrical safety and recently built or rewired houses should be relatively OK, but older premises and different supply and use regs in other countries may pose a greater risk.

Rob-T

john swift
22-04-2019, 08:14 PM
the devil is going to be in the detail

when my home was rewired a single RCCB was fitted to protect all circuits
(these days a split consumer unit would be fitted with 2 RCCB's)

so if I shorted a light fitting like you then the RCCB will trip depending on
1) which earthing system has been used
2) the total load taken by mains powered devices in the house

with a TN-C system with the earth & neutral combined
the imbalance the RCCB detects depends on the total load in the house

with a TN-S system the imbalance the RCCB detects will be a combination of the total load in the house and the volt drop along the neutral in the 3 phase cable buried under the road

John

Clive S
22-04-2019, 09:30 PM
Then there are the mcb and rcd combined in one unit called a RCBO :cool: