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Should I electrically bond the pier

Bonding Earthing Ground Loop

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#1 nakbrooks

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Posted 20 May 2013 - 11:59 AM

[edited to make my question a bit clearer]

 

A bit of advice please concerning bonding and grounding.

 

I will be mounting the various observatory equipment (computer, DC power supplies, etc) in two steel cabinets near the pier.  The pier itself is a steel DDM85K equatorial pier (for a DDM85XL mount).

 

(1)  I intend to electrically bond the two cabinets and their doors to each other.  Should I also bond the pier to the cabinets to make sure that everything that the equipment is mounted on is at the same potential?

 

(2)  Should I then ground the bonded cabinets/pier (by connection to the ground conductor of the main electrical supply to the building).  They will already be imperfectly grounded as they will be bolted to the concrete observatory floor so I'm not sure if grounding them to the building ground conductor makes a ground loop more or less of a problem.

 

Thanks

 

Nigel


Edited by nakbrooks, 20 May 2013 - 02:19 PM.

Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
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#2 Bernd_Eppinger

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Posted 20 May 2013 - 05:26 PM

Hi, Nigel,

 

I think the following should be considered:

  • Lightning:

    If your steel cabinets can be reached by lightning, then I would try to get an Earth-connection for them that is as good as possible. The ground conductor of the main electrical supply can be a good Earth connection, if the conductor diameter is big enough, especially because it is the same potential which your electronics uses. But I am not an expert for lightning prevention regulations and the required wire cross-sections, especially not in Britain, so you may better ask your local electrician.

    I have also made the experience what happens if a lightning flash comes down in 200m distance: in the case I have seen, all cables, especially loops, picked up the electromagnetic field caused by the lightning. The resulting currents evaporated copper conductors at various PCBs of my grandmother's telephone infrastructure. So my recommendation would be to lay-out cables that are connected at both ends close together to keep the area that can pick up the magnetic field small. Even better, but more complicated, would be to put these cables into a common grounded metal tube. Example: the USB- and the power-cable between the mount and the steel cabinets. In this case, it would make sense to connect the aluminium body of the mount (or the pier, if it has electrical contact) to one end of the tube.

    If your dome is made from metal and well grounded, and if your cabinets and cables (and the mount of course) are all inside the dome, then I think lightning should be less dangerous, except for the cables that enter the dome (e.g. mains supply, network/Internet).
     
  • Radiated emissions:

    I haven't experienced problems due to radiated emissions with my DDM60 and my Laptop. If you fear them, it could help to connect your cabinets and the mount (pier) together and to ground them. Ideally, the cables between mount and cabinet should be as close as possible at the ground conductor, like for the lightning case.
     
  • Ground Loops:

    To answer this, it would be necessary to know all your cabling. I don't think there is a general answer, I am sorry.

    But you can make your own thoughts. Problems with ground loops typically occur when there is a cable that carries some current in parallel with a cable that carries a sensitive signal, and if one of the conductors of the power cable (usually the ground conductor) is directly or indirectly connected to the sensitive signal cable at both ends.

    As an example, if your computer and your mount power supply both have a connection between ground and the protective conductor of the wall outlet, then there is a connection between the USB cable and the power cable via computer, power-cord, wall-outlet and mount supply. Inside the mount (at least for my DDM60), they are connected, too. Now if there is a voltage drop on the power cable (e.g. if it is long and thin), then there will be also a voltage difference between both ends of the USB cable. Although this will cause some current through the USB-cable's ground connector, this current will not be able to eliminate the voltage difference. The remaining voltage difference, if still big enough, can disturb the USB signal. If your house has an old-style cabling where zero-conductor and earth-conductor use the same wire, then you get the same effect between your computer and your mount power supply, this time causing an AC voltage difference between both USB ends.

    There are two different strategies against ground loops. You can break them by inserting something into the loop which blocks the loop current. If e.g. in the above example, the mount power supply is replaced by an isolated supply that doesn't have a connection between protective conductor and output, then the loop is interrupted and the USB cable will be able to force nearly the same ground potential at both sides. High frequency problems of ground loops can sometimes be solved by ferrite beads or common mode filters.

    The other strategy is to try to force equal potential between two sides of a signal cable by additionally connecting the grounds of both sides by a thick and low impedance conductor. This conductor should be laid out close to the signal cable to avoid creating a loop that picks up magnetic fields (or lightning, see above). I think that is what you are trying by grounding the pier. To be effective, there should also be a connection between the pier and the mount's ground (is perhaps anyway there, can be measured), and a direct connection between the cabinet and the computer ground.

 

Best regards,

 

Bernd



#3 nakbrooks

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Posted 20 May 2013 - 06:21 PM

Thanks Bernd for that very comprehensive reply, it will take me time to digest it.

The dome is glass fibre and located about 100m from the nearest building. All the cables to the dome are underground. Inside the dome all cables are inside steel protection (either the cabinets or the pier). The connections between the cabinets and the pier are underground.

All of the power supplies are industrial (Siemens SITOP) well-regulated units with adjustable output to permit compensation for voltage loss if necessary. All PSUs will be separately protected with circuit breakers.

There are some drawings and pictures in the photos section of the forum if you are interested.

I will go through your notes in detail but it looks like I should bond everything together and ground it.

Thanks again.

Nigel
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Stratis Observatory, Hautes Pyrénées, France.
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#4 Bernd_Eppinger

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Posted 21 May 2013 - 07:18 AM

Hi, Nigel,

 

Sorry, I was not aware of the drawings when I wrote my reply.

 

As the cabinets are directly underneath the pier, and as you lay out your cables to the mount/telescope inside the pier, I think that electrically connecting the pier and the cabinets makes up a good shielding and hence a good protection against nearby lightning. Personally, I would additionally ground it. The cables from the weather-sensor that run into your computer cabinet may still need some form of extra lightning protection.

 

As the dome is made from glass fibre, there seems to be no protection against direct lightning into the telescope / mount / pier. Or does the dome have a lightning conductor?

 

As all cabling distances between power, computer and mount are short, I wouldn't expect ground loop problems there. Besides, the datasheet of the PSU100M 24V/20A (6EP1336-3BA10) says: "Potenzialtrennung primär/sekundär: Ja", so I would assume that the outputs are electrically isolated.

 

As far as I can see, the data cables (USB & Ethernet) to the house are optical, so they neither contribute to ground loops, nor can they conduct lightning into your cabinets. Placing them underground is also a certain protection against direct lightning into the cables.

 

 

Best regards,

 

Bernd



#5 nakbrooks

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Posted 21 May 2013 - 07:32 AM

Thanks again Bernd.

 

There are copper data cables to the main building from the observatory (two Cat6 shielded cables) but these also go underground.

 

For the mast, I plan to add surge suppressors to the PoE cables to the camera and Boltwood; I'm not sure about the Unihedron - surge suppression may be built-in to the controller box, I need to check.

 

I need to think about lightning conductors.  Adding one to the dome may be difficult (how do I ground it with the dome rotating)?  Maybe I could add one to the mast which is only 3 metres away from the dome?  On the other hand, I wonder whether a lightning conductor may actually attract lightning to the observatory area and cause more problems than it solves?  There are tall trees within 50m and a building within 100m which are probably more likely to attract lightning than the dome which is only 3.7m high and the mast which is only 3.5m high.

 

I will ask around in the village.  I think the church (400m away) has a lightning conductor but I don't think any other buildings in the village have, so I will see what problems they have had in the past.

 

Thanks again.

 

Nigel


Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
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#6 nakbrooks

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Posted 21 May 2013 - 07:37 AM

One other thought.  You will see from this CAD drawing (http://forum.astrosy...2-cad-internal/) that there is a heavy steel ring that supports the dome.  Maybe I could use this as a lightning conductor  would that work if it were bonded to a conducting stake in the ground (soil is heavy clay).  I will also ask my electrician for advice. 


Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
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#7 Bernd_Eppinger

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Posted 22 May 2013 - 09:55 PM

Hi, Nigel,

 

some comments:

 

 

I wonder whether a lightning conductor may actually attract lightning to the observatory area and cause more problems than it solves?

 

I have found some methods in Wikipedia to estimate the amount of required lightning protection and its effectiveness. Unfortunately only in German, as there seems to be no equivalent English entry. At least the Google-translation is mostly understandable, although Google's English is partly terrible.

 

Here is the German Link: http://de.wikipedia....iki/Blitzschutz

Translation: http://translate.goo...iki/Blitzschutz

 

If you look at the chapter called "Rolling sphere method", you can see that even for the weakest lightning protection class IV, the maximum distance for which a lightning catcher can give protection from direct lightning is 60m, but only if your dome is 0cm high (unrealistic) and if the catcher is at least 60m high. For your 3.7m high dome, I would do the following math:

 

Sphere with r=60m touching a vertical catcher at x=0 and the floor y=0 (dimensions in meters):

  (x-60)^2 + (y-60)^2 = 60^2;  (Pythagoras)

  => x= 60 - sqrt(60^2 - (y-60)^2);  (the other solution with opposite sign of the sqrt() makes no sense).

 

For your dome, y=3.7m. This results in x=39.256m. Anything farther away than this will not protect your dome according to the weakest lightning protection class IV. And even if there's something in exactly this distance, it must be at least 60m high. Needless to say that the trees in 50m distance, your house in 100m distance and the church in 400m distance offer no protection of class IV.

 

A similar calculation shows that a catcher on your dome (or the dome itself) will attract lightning from a radius of ca. 21m, if class IV is assumed and everything around your dome is flat (which is not the case: fence, mast).

 

 

 

I think the church (400m away) has a lightning conductor but I don't think any other buildings in the village have

 

I don't know the law in France, but in Germany the law says you need lightning protection if a lightning strike could either easily happen or have severe consequences (http://de.wikipedia....und_Blitzschutz). I think if a house burns down, that could be considered as a severe consequence. Anyhow, I am not aware of any modern house in Germany that doesn't have lightning protection.

 

 

 

Adding one [lightning conductor] to the dome may be difficult (how do I ground it with the dome rotating)?

 

There is a heavy steel ring that supports the dome.  Maybe I could use this as a lightning conductor

 

As I am no expert for lightning, I would recommend to check the following with your electrician. Besides, I don't know the details of your dome.

 

Depending how the rotating dome is supported: if the bearing is massive and entirely electrically conducting (e.g. steel), I could imagine that a lightning conductor on the dome could be connected to the rotating part of the bearing, and that the fixed part of the bearing could be connected to the earthing system. If I had equipment for some 10000 EUR inside the dome, I would attach a grid of lightning conductors instead of a single one on the outside of the dome in the hope that they serve as a Faraday cage and reduce the field strength inside the dome in case of a direct or nearby lightning strike.

 

How to build the earthing system: there are several links in the article, but I think you should discuss it with your electrician. Reading the article, I have come to the conclusion that the Earth connector of the mains supply on its own is not sufficient. However, I still think it could make sense to connect the mains Earth to your Earthing device in an attempt to reduce the voltage differences between floor and mains in case of lightning.

 

I can't tell you if a conducting stake in the soil is sufficient. I assume it depends on its size and depth. Wikipedia says such devices ("Ground rods") may well be driven 9m or more into the soil ("nine feet" is a translation error by Google).

 

 

 

There are copper data cables to the main building from the observatory (two Cat6 shielded cables) but these also go underground.

 

Even if they are underground, I would think about adding a lightning arrestor where these cables enter you dome or your metal cages (if that exists for Ethernet), or at least ground the shield well. The electrostatic field of lightning can extend several meters below the Earth surface, because soil is not a perfect conductor.

 

Or you may consider an optical fiber cable instead of CAT6, because it is non-conducting and doesn't conduct lightning to your electronics. Besides, as far as I know, transmitting high data rates (>1Gbps) over large distances (100m?) is easier when fiber is used instead of copper.

 

 

Best regards,

 

Bernd



#8 nakbrooks

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Posted 22 May 2013 - 10:24 PM

Thanks again Bernd. Food for thought!
Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
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