Shielded Cable and grounding

[andy257]

Hi All,

i would like to know in a system where there are AC voltages and DC voltages which ground should a shielded cable be connected to, DC Ground or AC Ground (Mains earth)?

E.g.

Power supply connected to a torque transducer and a DAQ card measuring the signal output from the torque transducer. The cable connecting the DAQ card and the transducer has +ve and -Ve cables plus a shield braid. Where do i connect the braid? The box (chassis) which this kit is kept in is earthed because the PSU sits inside it.

Therefore i see two choices

Connect the braid the the chassis which is earthed via the mains input
Connect the braid to the GND of the DC PSU

Can someone help me understand this.

thanks

 

[Sceadwian]

It should be earth ground, but the DC PSU should also be connected to earth ground as well if that's the case.

 

[JimB]

If the DAQ card has a place for a screen, connect it there.

Otherwise I would be inclined to connect it to the chassis (case), also ENSURE that the 0v line of the PSU connects to the chassis.

JimB

 

[KeepItSimpleStupid]

OK, here is the deal.

For transducers, the shield should be connected to earth on the signal source side only. The shield reduces RFI or Radio Frequency Interference. Twisting reduces EMI or Electromagnetic Interference. So, running signals close to power lines benefits by twisting the signal and the return line together. A signal magnetically coupled into the cable will try to cancel due to the twisting (Magnetic coupling).

You can't always follow the suggestions, but in general shields should be connected only at one end. It's not true with 75 ohm coax for TV for instance or Phono plugs for stereos. Stereo's should place a low value resistor to ground so that nasty ground loops do not occur. Professional systems use a "balanced" system with an overall shield which is a twisted pair shielded system.

You always strive to have all of the returns terminate at one place. In places such as a hospital or Radio station, there will be two grounds carried to the building ground. One will be "protective ground" and the other will be "the reference".

Suppose that there was say an MRI which developed a ground fault. It could raise the potential of ground on anything on that feeder. If the fault was returned to the building ground it would have no effect.

In our homes we risk damage, but the potential for damage is low. A lightning strike to ground inside the house will cause damage, A lightning strike to a phone line will not. Whole house surge suppressors have a chance to protect against lightning if say the line leading from the transformer to the house was directly hit.

 

[westom]

i would like to know in a system where there are AC voltages and DC voltages which ground should a shielded cable be connected to, DC Ground or AC Ground (Mains earth)?

First learn what grounds exist. If a better design, then it has an analogue ground, digital ground, chassis ground, and AC safety ground. You don't have an earth ground (at the receptacle) even though a UK standard mistakenly calls earth ground what is only a safety ground. Earth ground is where an electrode enters the earth.

Each ground may be interconnected. And all are electrically different.

As KeepItSimpleStupid has accurately noted, a shield is typically connected only at one end. Because a best shield does not carry current. Whereas a conductive ground (ie coax cable) must be connected at both ends. Does not provide the superior isolation provided by a non-current carrying shield. But is better than nothing.