Telescoping safety earth?

[abbey road d enfer]

Hi all,
Considering an electronic instrument consisting in a power amp, preamp, sound generator and keyboard, is it possible to connect the safety earth to the power amp chassis, then telescope it to the preamp, and then to the sound generator, and finally to the keyboard keyboard, in a daisy-chain fashion, or is it necessary to have direct wires from the earth point to each separate section?

 

[JohnRoberts]

EGC equipment grounding conductors are a power thing more than an audio thing, but the two worlds overlap.

UL has current carrying (ground bonding) rules for I/O terminations on audio gear that are labelled as "ground". IIRC it had to carry several tens of amps fault to safety ground with only single digit voltage rise.

JR

 

[Bo Deadly]

Are these separate devices with their own chassis?

Does signal 0V connect to safety earth and if yes, where?

If 0V is separate from safety earth, then I don't think it matters but because safety earth is about providing a safe path for rogue currents, it would make sense that safety earth is handled in the same way power is handled. So if you're daisy chaining power, you should daisy chain safety ground.

But your question is a little light on details.

 

[JohnRoberts]

I've told this story before but back when I was designing fixed install gear at Peavey, I had a small utility amp fail UL testing because the ground bond testing evaporated a PCB ground trace. I was told by my in house agency guy that I could just relabel the connection something like 0V instead of "ground" and not have to pass that test. I decided instead to beef up the PCB trace to pass the ground bonding test. It seemed like the right thing to do.

JR

 

[abbey road d enfer]

Are these separate devices with their own chassis?

Yes.

Does signal 0V connect to safety earth and if yes, where?

Yes, only in the amp chassis where the mains connector is (IEC type).

But your question is a little light on details.

See attachment.
In daiy chain mode, safety earth goes from amp chassis to preamp chassis, then another wire goes from preamp chassis to tone gen chassis and anothe goes from tone gen chassis to KB chassis.
That's one earth wire in each loom.
In star mode safety earth goes from amp chassis to each otehr chassis with separate wires. That means 3 earth wires in A", 2 in B" and one in C".
Does the obligation of continuity accept connections or only a one-piece wire?
Mains is only in the amp chassis.

 

[ccaudle]

Safety earth is a power concept, so if mains is only in the amp chassis, safety earth should only be needed in the amp chassis. The other chassis have no way that mains voltage could short to the chassis, so safety earth would provide no function.

 

[zamproject]

The other chassis have no way that mains voltage could short to the chassis,

Just speculating, but what about a power supply failure that send main at secondary and to other modules.
Probably there will be return with ground/0V, then to earth where they joint, but this is probably not able to carry the same safety as independent frame/earth bolted wire whatever star or string ?

 

[Matador]

Safety earth is a power concept, so if mains is only in the amp chassis, safety earth should only be needed in the amp chassis. The other chassis have no way that mains voltage could short to the chassis, so safety earth would provide no function.

If B+ shorted to the chassis on the last module, and there was no safety earth path back to the IEC, then what prevent possibly lethal voltages from being present on that last chassis (for example, what if wire C was disconnected or broken)? This might go doubly if 0V is completely isolated from the chassis in each module.

 

[abbey road d enfer]

Safety earth is a power concept, so if mains is only in the amp chassis, safety earth should only be needed in the amp chassis. The other chassis have no way that mains voltage could short to the chassis, so safety earth would provide no function.

That's more or less the idea I had originally, but still, earth continuity is valuable on the point of view of EMI/RFI.
So it looks like telescoping earth is acceptable.

 

[abbey road d enfer]

If B+ shorted to the chassis on the last module, and there was no safety earth path back to the IEC, then what prevent possibly lethal voltages from being present on that last chassis? This might go doubly if 0V is completely isolated from the chassis in each module.

There is no doubt that all chassis must be earthed. However the constraints are not so big as for mains voltage. A smaller conductor should be sufficient.

 

[Matador]

There is no doubt that all chassis must be earrthed. However the constraints are not so big as for mains voltage. A smaller conductor should be sufficient.

Sorry, I edited after you had responded.

If every module were to have a PSU, then you would (be forced to) implement the star, right? As in, every module would have an IEC plug and each would have a dedicated safety ground leading back to whatever all of the modules were plugged into.

Now you are using the those voltages in each module, so why would safety earth be treated any differently?

I think in my mind, it would come down to how likely the modules were to be moved from location to location, and how likely it is that someone would forget to connected the daisy chain. Again, with power in each module, this sorts itself out.

 

[JohnRoberts]

Funny (not funny ha ha ), my old house is wired with only 2 conductors, so no 3rd wire safety ground. I have since installed GFCI outlets for every one near water (bathroom/kitchen/laundry). The most human safety drama I encountered personally was when the heat element in my hot water heater rusted through and energized my hot water feed, giving "hot water" a whole new meaning.

While this is not to electrical code, I manually wired the hot water tank and copper plumbing to my fuse panel ground, and I even ran an additional wire to my kitchen outlet providing a true ground.

JR

 

[moamps]

There is no doubt that all chassis must be earthed. However the constraints are not so big as for mains voltage. A smaller conductor should be sufficient.

Which voltage is a greater danger to humans, mains of 230VAC/16Afused or B+ of 350VDC/100mAfused?
Also, the safety earth wire must withstand all fault currents, including the short-circuit current of the heating voltage, if it is not floating.

In my country, any voltage higher than 50V is considered dangerous.

To answer the initial question, if it is a fixed setup, I would prefer star-earthing, if it is a setup that needs to be connected and disconnected frequently, I would prefer telescoping. Basically, safety earth should always be connected first and disconnected last, and this cannot always be ensured with separated cables.

 

[Bo Deadly]

If safety earth and signal 0V are separate, then there should be no impact on the audio path.
It seems to me that the issues being considered are purely related to safety and secondarily to shielding from EMI.
If there is no mains in devices other than the power amp, then technically you don't even really need earth ground in those devices.

So electrically I don't see any real difference in the two scenarios you are considering.
But obviously they are very different physically and the second scenario is just ugly.
Why have separate conductors if you don't need them?
In fact, it could be dangerous because presumably it would be possible to disconnect the wires when setting up the instrument.
If the operator forgot to connect the safety earth of one device and B+ touched the chassis, they might get shocked.
Frankly I'm concerned that someone with your experience would even consider doing it the second way.
Have you taken your performance pill today?

From the top, the way I would think about this is that whatever current the chassis is supposed to shunt should return on the same cable that supplied it.
It seems to me the primary source of current in the chassis would be from a fault that allowed B+ or heater to touch the chassis.
So the chassis of each device should be connected the the cable shield which should terminate at the power supply where it senses an over-current condition.
For a linear supply, this would be the 0V end of the transformer (which will be reflected into the primary which should have a fuse).
For an SMPS (12VDC heaters?) this would be the 0V output.
The cable shield and it's interconnects just have to handle enough current to blow the fuse on the supply or to trigger over-current protection of an SMPS.
So even something that used a drain wire would be ok.

EMI is secondary to safety and can be handled with a separate conductor from said 0V at the PS to the chassis bolt in the power amp where it then goes on to the safety earth pin of the IEC connector.

At least that's how I would do it based on the information provided.

 

[sahib]

Hi all,
Considering an electronic instrument consisting in a power amp, preamp, sound generator and keyboard........

English is also my second language but I hope you do not mind me making a small correction here.

"Consisting in a power amp..." implies that the preamp, sound generator and keyboard are housed inside the power amp. I think you mean consisting of a power amp, preamp, sound generator and keyboard.

Also in your post #2 I am not sure what you are referring to with "chassis" in terms of the preamp, sound generator and keyboard. Do they have their own metal housing?

 

[ccaudle]

If B+ shorted to the chassis on the last module, and there was no safety earth path back to the IEC, then what prevent possibly lethal voltages from being present on that last chassis

If B+ shorted to chassis, there is no reference to mains input (B+ is derived from an isolated secondary winding), so even if there was a safety earth, no current would flow in the safety earth because there is no complete circuit which includes mains entry safety earth and the B+ supply.
Assuming the B+ return path was connected to chassis, then you would still have a short circuit from B+ through chassis, through return path to transformer secondary, and you would either blow your secondary side fuse if that existed, blow your transformer primary side fuse, or if the winding resistance was high enough and the primary size fuse was sized too high, just circulate current, getting the wiring and possibly sheet metal hot.
That would be independent of mains safety earth connection, though, and would apply even if you had a battery powered supply generating B+ and the entire system was isolated.

Now you are using the those voltages in each module, so why would safety earth be treated any differently?

But mains voltages are not being used in each module (according to post #5, "mains is only in the amp chassis"), so safety earth is not being treated differently. Safety earth follows mains, no mains in the other modules, so requirement for safety earth does not apply.

I do hesitate based on the question from zamproject in post #7, namely how to handle a primary side to secondary side short in the transformer.
That should typically be handled by using transformers with safe construction. I do not know what ratings numbers would indicate that (e.g. UL recognition), but the term "double insulated" is a general description of transformers with two layers of protective insulation around the primary side to prevent a single fault from allowing dangerous contact of primary winding voltage to anything else (secondary winding, frame, chassis, etc.).

 

[Newmarket]

Is the term "telescoping" correct here. Ime it applies to overlapping but not connecting cable screens. But here the question seems to be single cables going to a "Star Point" Vs daisy chaining PEC connections ?

 

[zamproject]

To answer the initial question, if it is a fixed setup, I would prefer star-earthing, if it is a setup that needs to be connected and disconnected frequently, I would prefer telescoping. Basically, safety earth should always be connected first and disconnected last, and this cannot always be ensured with separated cables.

That make sense to me !?!

So electrically I don't see any real difference in the two scenarios you are considering.

I agree there is not, I think the concern here is only safety.

Like should all earth connection(for non floating chassis) get individual return to main earth point/IEC connector (star distribution) or is it -allowed- to have pass through connector/distribution (mechanically speaking) like a garland ?

Frankly I'm concerned that someone with your experience would even consider doing it the second way.
Have you taken your performance pill today?

I'm sure Abbey know what he's asking for no need for pills

 

[abbey road d enfer]

Frankly I'm concerned that someone with your experience would even consider doing it the second way.
Have you taken your performance pill today?

If it was only me, I know what I would do, but here I'm dealing with regulations, so common sense is not enough.

 

[abbey road d enfer]

"Consisting in a power amp..." implies that the preamp, sound generator and keyboard are housed inside the power amp. I think you mean consisting of a power amp, preamp, sound generator and keyboard.

Correct. "Consisting in" is a malapropism based on french syntax.

Also in your post #2 I am not sure what you are referring to with "chassis" in terms of the preamp, sound generator and keyboard. Do they have their own metal housing?

Yes, they do.