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Old 25th Aug 2022, 3:20 pm   #1
lesmw0sec
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Default To bond or not to bond?

I am constructing a 13.8V supply for a transceiver (Linear mode). It occurred to me that it might be worth bonding the negative line of the output to earth to prevent a shock in the unlikely event that the transformer may fault with a primary to secondary short.

I can see a disadvantage with this, given that the transceiver would normally have its own earth for RF purposes and such an arrangement may give rise to earth loops or PME issues. In the past I have left the output floating.

Would anyone know what the method adopted by the commercial chaps is?
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Old 25th Aug 2022, 4:18 pm   #2
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Default Re: To bond or not to bond?

From the safety point of view, you are designing an Extra Low Voltage supply, defined as less than 50 VAC or 120V DC ripple free, so your looking at three basic types:

SELV - Separated Extra Low voltage where the output cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits. You will need a high-grade isolating transformer for this. (BS EN 61558, formerly BS3535).

PELV - Protected Extra Low Voltage - in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, except earth faults in other circuits. This ciruit may be earthed on one side of the ELV.

FELV is simply one that works at ELV but does not necessarily have the level of protection of SELV or PELV.

SELV & PELV should be insulation tested at 250VDC to earth and have a minimum insulation resistance of 0.5 megohms, although I would be concerned unless it was much higher.

You find some diagrams and description here: https://learnelectrics.com/what-is-selv-and-pelv/

It looks like you chose SELV in the past, you just have to ensure that the insulation test can be met to be sure it is a valid SELV system.

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Old 25th Aug 2022, 5:07 pm   #3
G6Tanuki
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Default Re: To bond or not to bond?

The commercial supplies I used to work with [typically 24V@40A] had the DC output floating, but with strapping-options on the output terminals so you could - if needed - link one side of the output to station ground.

This sometimes caused issues with the 'remote sense' on the regulator which could get to 'see' stray circulating currents in the negative wiring.

Equally, quite a bit of gear I've worked on have the regulator-transistors for the PSU implemented so the cases [collectors] of the transistors are grounded and the regulator operates on the negative side - this avoids the need for insulating washers between the heatsink and the series-pass transistors but does mean that the 'negative' side of the smoothing-capacitor-stack and the transformer windings sit at a variable voltage above station ground which can lead to fault-diagnostic difficulties when done by someone unfamiliar with the idea.
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Old 25th Aug 2022, 5:42 pm   #4
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Default Re: To bond or not to bond?

commercially made linear power supplies will either have a double wound transformer with an earthed screen winding (older sort) or else a split bobbin transformer with the pri and sec windings seperated with reinforced insulation between the two (modern sort). Internal layout likewise. Either way it would be safe to keep the secondary floating.

I remember the Mk1 version of the 'radford labpack' (the battleship grey one with round knobs) was wired in such a way that part of the low voltage raylamp wiring was loomed together with part of the mains wiring. If a student tried shorting the raylamp supply out, the wiring could get hot enough to melt into the mains wiring, thus breaching mains - output insulation. Later ones routed the cabling differently.

The IEE/IET description of SELV supplies doesn't explain what is meant by 'seperated' when an SELV lighting transformer fills with rain water and mains gets onto the 12V output that way.
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Old 26th Aug 2022, 8:32 am   #5
lesmw0sec
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Default Re: To bond or not to bond?

Thanks for feedback. The transformer I have is an enormous toroid. It was salvaged from what was originally a variable supply. It does have a makers name & model number, so I can look up its specs. I would be quite happy with the types where the bobin separates the primary from the secondary, but always a bit suspicious where the windings are layered.
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Old 26th Aug 2022, 11:08 pm   #6
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Default Re: To bond or not to bond?

Mm, I guess that's a bone of contention. I suppose toroidals are fairly easy to insulate between pri-sec, but I cant think of a floating output PSU that uses one. I've seen a few with interwinding screens and lots of toroidals in my world, high power PA amps, use them but with a centre tapped earthed secondary or output terminal. I wonder what the transformer winder experts make of it.
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