Question for grounding of the electronics to Faraday cage

[goodphy]

Hello.

Let's say there is a instrument needed to be EM wave-shielded in High voltage discharge lab. Building Faraday cage becomes necessary and the instrument is moved to inside the cage. The cage is grounded to lab ground and the frame ground of the instrument (it is also signal ground for the instrument here) is connected to metal piece of the cage.

We suspected that EM wave from gas discharge makes noise which can flow along a ground wire. (so also Faraday cage I think) In this sense, EM wave still disrupts shielded instrument inside the cage via ground connection.

Could you tell me that my reasoning is right? If so, is it typical case in EMI environment and how can I removes ground noise to the instrument? It may need to be noted that our lab is 2nd floors and total length of wire from lab ground to Earth is to be very long thus huge ground potential rise with respect to Earth in transient time (~discharge time) is expected.

 

[RaulTheUCSCSlug]

How big is this instrument that you are talking about? I know that small farraday cages don't need to be grounded, but I think it's different for larger Faraday cages.

Couldn't you just create another cage around the cage you already have and put an insulator between them, wouldn't that help since there won't be a similar ground connection?

 

[Drakkith]

Can you just put a filter on the equipment's ground cable?

 

[goodphy]

Size of cage is about double size of typical delay generator like 400 (mm) * 350 (mm) * 150 (mm). Installation of the filter to the ground cable would be good idea. Can you recommended me a filter for ground wire?

 

[tech99]

Hello.

Let's say there is a instrument needed to be EM wave-shielded in High voltage discharge lab. Building Faraday cage becomes necessary and the instrument is moved to inside the cage. The cage is grounded to lab ground and the frame ground of the instrument (it is also signal ground for the instrument here) is connected to metal piece of the cage.

We suspected that EM wave from gas discharge makes noise which can flow along a ground wire. (so also Faraday cage I think) In this sense, EM wave still disrupts shielded instrument inside the cage via ground connection.

Could you tell me that my reasoning is right? If so, is it typical case in EMI environment and how can I removes ground noise to the instrument? It may need to be noted that our lab is 2nd floors and total length of wire from lab ground to Earth is to be very long thus huge ground potential rise with respect to Earth in transient time (~discharge time) is expected.

The ground wire from outside must not enter the cage at all; it should be connected to the outside surface of the cage. It might be worth using a solid metal plate in that vicinity. The ground wire from the equipment must not go outside the cage at all. Are you a bringing power lead or any other wire into the cage? If so, it must be filtered and bypassed to the cage. Keep the filter in a screened box. The size of mesh will decide the maximum frequency at which the cage is effective. Joints and cracks in the cage, including the door, can act as slot antennas, so joints must be continuous. Make sure there is no radio reception inside the cage.

 

[goodphy]

The ground wire from outside must not enter the cage at all; it should be connected to the outside surface of the cage. It might be worth using a solid metal plate in that vicinity. The ground wire from the equipment must not go outside the cage at all. Are you a bringing power lead or any other wire into the cage? If so, it must be filtered and bypassed to the cage. Keep the filter in a screened box. The size of mesh will decide the maximum frequency at which the cage is effective. Joints and cracks in the cage, including the door, can act as slot antennas, so joints must be continuous. Make sure there is no radio reception inside the cage.

Sounds very useful for me. Yes, I need to bring power from outside instrument inside the cage. Regarding this issue, I have a question. The power code typically has not only current loop wires (maybe called live and neutral I think.) but also ground line. Is normal power line filter also able to block the noise flowing via ground wire of the power code?

 

[tech99]

Sounds very useful for me. Yes, I need to bring power from outside instrument inside the cage. Regarding this issue, I have a question. The power code typically has not only current loop wires (maybe called live and neutral I think.) but also ground line. Is normal power line filter also able to block the noise flowing via ground wire of the power code?

A problem with power line filters is that the ground wire will often pass straight through. This allows common mode noise currents to pass straight through into the effected equipment. To reduce this effect, a common mode choke can be used, all three wires being wound on to a lossy ferrite ring, or clip-on ferrites can be used on the cable. In the case of a good Faraday cage, if the ground wire connects to the outside of the cage, it should not be able to introduce noise, but it may still be worth adding a common mode choke external to the cage. EMI problems are very often caused by common mode currents, especially where the cable is long. Be careful that no cable is entering that you have overlooked.

 

[goodphy]

A problem with power line filters is that the ground wire will often pass straight through. This allows common mode noise currents to pass straight through into the effected equipment. To reduce this effect, a common mode choke can be used, all three wires being wound on to a lossy ferrite ring, or clip-on ferrites can be used on the cable. In the case of a good Faraday cage, if the ground wire connects to the outside of the cage, it should not be able to introduce noise, but it may still be worth adding a common mode choke external to the cage. EMI problems are very often caused by common mode currents, especially where the cable is long. Be careful that no cable is entering that you have overlooked.

Yes common mode noise (noise flowing through the ground and eventually reaching signal wires or its opposite direction) is exactly my concerns. Ferrite toroid seems good idea but I need to which range of frequency it works? Our noise is peaked 1 MHz and 20 MHz. Is Ferrite good for this relatively low frequency in RF?

 

[tech99]

Yes common mode noise (noise flowing through the ground and eventually reaching signal wires or its opposite direction) is exactly my concerns. Ferrite toroid seems good idea but I need to which range of frequency it works? Our noise is peaked 1 MHz and 20 MHz. Is Ferrite good for this relatively low frequency in RF?

The following link might be useful. It suggests winding several turns on to a stack of two big Fair Rite Type 43 ferrite rings for the lower frequency and one of Type 73 for the higher. I have occasionally used as many turns as possible on a big clip-on ferrite core.
http://rsgb.org/main/technical/emc/using-emc-filters-and-ferrites/

 

[meBigGuy]

THis has info on the installation of power line filters for screen rooms ( as well as filters)

http://www.rficorp.com/downloads/Filtron_Filters_Catalog_-_Low_Res_-_May_2011.pdf