PLC/electronics and grounding? Two earths necessary?

cerastes

My enquiry is about grounding for a PLC in a scenario where there is not a separate ground/earth to that of the power supply?
My understanding from reading up is that there may be a requirement for two earth's, but I've also read that one would work and may even be preferable.

Can a connection be made to a single earth for a ground connection where noise can be filtered? Or to prevent damage to electronics.

2011

Typically control systems have two different earthing systems:

1) The "dirty earth". This is a safety earth. The primary intention of this earthing system is to provide a return path for fault currents. It is also for equip-potential bonding. All extraneous conductive parts should be connected to this. This earthing system would have a lot of "noise" on it.

2) The "clean earth" or "instrument" earth. The design intent of this earthing system is to prevent interference on the cables for the instruments connected to the control system.

The clean earth system should be a complete separate earthing system that is connected to the dirty earth system at only one point. This should be a very low impedance connection. Normally the screens of instrument cables are left "floating" (tied back and insulated) at the instrument/ field end. These screens are then connected back to the instrument earth bar.

cerastes

MSo in a domestic scenario where I want to play around with a PLC? do I need to run a seperate earth as there seems to be a ground on the programming cable, or a sperate earth for the earth connection on the PLC? If the clean earth is still connected even only at one point does that still not introduce noise? Noted I've only domestic items to consider but still some motors from appliances and maybe noise from other electronics.

If it's a low impedance connection at one point only just a larger CSA or should any filter be inline to eliminate noise too?

In my case, I'm doing this at home, how much noise is there likely to be? Could I connect to the power (only) earth and do it that way or additionally add some kind of filter? To enable me to use the existing earth, might this cause damage to the PLC? Or disrupt it's functioning?
Looking to see if I can get a course, but doesn't seem to be an evening one I can find.

2011

cerastes wrote: »

MSo in a domestic scenario where I want to play around with a PLC? do I need to run a seperate earth as there seems to be a ground on the programming cable, or a sperate earth for the earth connection on the PLC? If the clean earth is still connected even only at one point does that still not introduce noise? Noted I've only domestic items to consider but still some motors from appliances and maybe noise from other electronics.

Interference / noise is more of an issue when there are long cable runs and high currents. This is unlikely to be much of an issue in a domestic

If it's a low impedance connection at one point only just a larger CSA or should any filter be inline to eliminate noise too?

In my case, I'm doing this at home, how much noise is there likely to be?

Negligible.

Could I connect to the power (only) earth and do it that way

That is what I would do. Connect from the incoming earth to an insulated instrument earth bar. Connect all screens (at the PLC end only) to this bar.

or additionally add some kind of filter?

I wouldn't bother.

2011

wefixtexas wrote: »

When an electrical fault develops in domestic applications, any metal parts that the user is in contact with may become live electrical conductors and cause an electrical shock. Adequate earthing of that metal part, however, can provide an unfailing circuit path away from the user to an earth ground, preventing the shock.

If the circuit is protected by a ground-fault circuit breaker, the fault will trigger the tripping of the breaker and remove both the current and the shock potential.

In industrial applications, the condition of earth resistance or leakage current testing may have been neglected, perhaps for a long time, because of the time consuming methods involved, ignoring existing earthing measurement predictive plans, or the non-availability of good testing instruments. This condition can be so bad that it can cause transformer failures and erratic meter readings at high tension feeders, etc., leading to major accidents.

Other than the protection of humans and equipment, the purpose of earthing is to provide a safe path for the dissipation of fault currents, lightning strikes, static discharges, EMI and RFI signals, and interference.

Prior to selecting the location of a substation or commercial sector, it is necessary to measure the soil resistance value

Regards wefixtexas

This has nothing to do with the OP's question.

cerastes

2011 wrote: »

This has nothing to do with the OP's question.

I've a habit of going on tangents myself, they can lead to intersting stuff I'd be thinking about anyway.
Any good resources for links for setting up plcs? My main concern is not damaging the unit, I'll print the manual and try interpret that in more detail after. I realised there is the earth in the unit and a fairly heavy ground lead from the programming cable I couldn't see in the manual online whe e this is meant to connect to so have to conclude its to the instrument earth normally,
also my laptop has an equivalent connection to the programming lead, I need to check but I think it's R's 232 both are in female, trying to determine if a straight through adapter of this in male/male could join my laptop to the programming lead

2011

cerastes wrote: »

I've a habit of going on tangents myself, they can lead to intersting stuff I'd be thinking about anyway.

The post by wefixtexas was about CPC's (circuit protective conductors). This is when an earth cable is used to complete the circuit back to ground for a flat current with the intend of causing the protective device(s) to operate. The instrument earth is to prevent noise / interference propagating to the signal cables.

Any good resources for links for setting up plcs?

You will find some good material on clean earthing arrangements:

Here, here, and page 3 here.

My main concern is not damaging the unit, I'll print the manual and try interpret that in more detail after.

Follow the manual and you will not damage the unit.

How long are your instrumentation cables?
How close are they to sources of interference?

gctest50

Originally Posted by wefixtexas

When an electrical fault develops in domestic applications, any metal parts that the user is in contact with may become live electrical conductors and cause an electrical shock. Adequate earthing of that metal part, however, can provide an unfailing circuit path away from the user to an earth ground, preventing the shock.


...........

Regards wefixtexas

2011 wrote: »

This has nothing to do with the OP's question.

its just a bot ?

http://www.brighthubengineering.com/commercial-electrical-applications/123608-grounding-methods-and-measurement/


When an electrical fault develops in domestic applications, any metal parts that the user is in contact with may become live electrical conductors and cause an electrical shock. Adequate earthing of that metal part, however, can provide an unfailing circuit path away from the user to an earth ground, preventing the shock.

cerastes

Initially I plan to make input and output connection right at the unit/very close to it.
Distance to items that might create electrical noise, electric shower upstairs, washing machine, dishwasher downstairs if motors created that, I'm wondering if there is anything from cfl items? General domestic stuff really.
Just to write ladder programme and test making manual inputs and test outputs.

2011

gctest50 wrote: »

its just a bot ?

Yup

2011

cerastes wrote: »

Initially I plan to make input and output connection right at the unit/very close to it.
Distance to items that might create electrical noise, electric shower upstairs, washing machine, dishwasher downstairs if motors created that, I'm wondering if there is anything from cfl items? General domestic stuff really.
Just to write ladder programme and test making manual inputs and test outputs.

I don't think you have anything to worry about. CFLs will be fine.

cerastes

And a cheap arc welder, never used it, I suppose if that was turned on ever, maybe I could disconnect the PLC as a precaution.