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Another Electrical Question - Earth Loops


hughgilhespie
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I am just finalising the design of my main DC power supply for my obsy. It is basically a fleabay 14 - 16 volt switchmode PSU that can provide lots of amps - about 40 as I recall. The fleabay PSU will be set to about 14.2 volts and then dropped to about 13.6 volts by a 10 amp rated diode. The drop in voltage is to allow the PSU to trickle charge a back-up 22 AH battery that is combined with the switchmode unit to give a UPS capability. This is so I can power down everything smoothly when we have a mains outage - fairly frequent here.

The output from this UPS box then goes to the pier and to a distribution box where it will - in the fullness of time - power the mount, scope, dew heaters, cameras, autofocus, USB hub, etc. The pier distribution box will shift voltage as required for the various outputs.

My question is about the earthing arrangements. Originally I was going to common up all the DC negative lines and the mains earth then run a two-core cable to the pier box. But, the output from the main switchmode box is floating, so I have the option of using a three core cable from my unit, DC+, DC- and earth. Then I can safely earth all the metalwork - pier, mount, etc. but leave the DC supplies floating. What is the best way to wire this to avoid earth loops?

Regards, Hugh

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Hi Hugh,

Switch mode PSU`s have a nasty habit of having 70V or more on their -ve output so it can either give you a shock (feels more like a vibration) or at some point it will end up getting earthed anyway with a PC usb socket or similar causing localised current flow.

To eliminate these issues I would earth the -ve rail at source but still use 3 wires to the pier and using the "earth" one to ground the mechanical bits seperately.

Alan

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Alan, thanks for that, sounds like a good way to go.

Per - I think the typical running current (mount tracking + one dew heater) will be about 4 amps so only 2.4 watts to worry about. The diode is a TO220 case that I can bolt direct to the aluminium chassis plate. I don't expect the peak current to exceed 8 amps, so less than 5 watts and that only for short times. Not too hot and a little heat is no bad thing to keep everything dry.

Regards, Hugh

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In a perfect world the DC wouldn't be referenced to earth.

That's half the benefit of using it. Low voltage and and no path to true earth (the general mass of the earth).

With switched mode PSU's there can be strange voltages due to filtering (sufficient to trip an RCD if there are multiple computers on one circuit in an office for example).

If, as Alien 13 says above, the output is at 70v relative to earth it is potentially lethal unless it collapses as soon as current flows.

The wiring regulations require that "touch voltages" be limited to no more than 50v because at that voltage enough current may pass through the human body to cause a fatal shock. This is the reasoning behind the 30mA rating for RCD's in domestic AC circuits.

Also, in a perfect world you would not run the domestic supply's earthing system to outdoor equipment, particularly TNC-S (PME) which is usually forbidden by power company regulations anyway.

This is because under certain circumstances the supply earth may be at significant potential relative to true earth.

So what to do!?

The DC would be best if fully separated (SELV), ie run the PSU via an isolating transformer.

Or as a minimum, knock an earth in rod at the observatory and connect the DC -ve to that. Because the only danger is from potential against the earth you are stood on. Exporting the domestic supply earth introduces risks that weren't there previously- unless there is other class 1 mains equipment in use already.

Disclaimer: This advice is worth what you paid for it!

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Being rural the overhead supply here does not have an earth - just line and neutral - the house earth is provided by an earth rod outside the kitchen window and enters the house near the meter box where it is connected to the ring main earth circuit.  Whenever I use electrical equipment outdoors I run it from an RCD.  The observatory supply also has its own RCD.

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Being rural the overhead supply here does not have an earth - just line and neutral - the house earth is provided by an earth rod outside the kitchen window and enters the house near the meter box where it is connected to the ring main earth circuit.  Whenever I use electrical equipment outdoors I run it from an RCD.  The observatory supply also has its own RCD.

Thats the typical TT system.

The neutral is earthed back at source and the property has an earth rod.

The earth rod resistance with a 30mA RCD should be no more than 1666ohms.

If there are metal water or gas pipes these should be connected to the earth rod, I believe 10mm cable should be used.

The reason there is no earth.

The 'company' cannot guarantee the overhead earth connection, it could easily break or in some cases be stolen.

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