(Circuit details removed by moderator)

I am doing a large aquaponic greenhouse so require a number of these circuits. (60 to 70 modules - so 12 to 14 circuits).
I have searched and read through a lot of this forum and the moderators cut off discussion like this due to safety concerns. Everyone says to use a current limiting power supply on each module or at least suppling through an isolation transformer. Both options are very expensive and complicate the system a lot.

My question is: why is higher voltage DC only safe when supplied through an isolation transformer when it is perfectly normal to series feed 115VAC modules with the only current limiting done with the circuit breaker?

Thanks in advance for the education!

 

It's because line voltage neutral is connected to earth, so any contact with a hot wire, if you are touching (or standing on) earth, will result in an electric shock.
A transformer provides isolation from the earth ground so you have to contact both the hot side and circuit common to get a shock.

One way to minimize the danger is to use a GFIC plug, such as this, for the circuit.
That way you can still get a shock but it shouldn't kill you.
(I tripped a GFIC once by accidentally touching a hot wire in my garage and you definitely get a jolt you will remember, but I lived. )

 

1. transformers do NOT provide a DC output.

2. high voltage is equally dangerous regardless if referenced to ground or not - it is just that risks get somewhat shifted from one set of concerns to another which if done correctly can statistically appear safer. if isolation was always safer, utility company and national electrical code would not insist on grounding one terminal of the secondary.

3. are you familiar with breakdown voltage of LEDs? you do realize that they are still polarized devices and therefore work as rectifiers in AC circuit? what do you think will happen when capacitor gets charged?

4. you cannot get current limiting without voltage drop. in other words, connecting 5x32VDC loads in series to power them from 115VAC means that your capacitor is going to be some rather odd device to perform as you intended. you can have 4x modules in series, so that current limiting circuit has some voltage drop. you can also add another 4x modules, also in series but connected in anti-parallel with first set of modules. that means 8 modules and one current limiting device and no issue with polarization and ... you get circuit that is far more tolerable to slight line voltage fluctuations.

 

re you familiar with breakdown voltage of LEDs? you do realize that they are still polarized devices and therefore work as rectifiers in AC circuit? what do you think will happen when capacitor gets charged?

That brings up a good point.

To operate from AC you would need a bridge rectifier for the LEDs with the current limiting capacitor in series with the bridge AC input.

Also to limit the turn-on current surge, which could zap the LEDs, you need some resistance in series with the capacitor.

 

1. transformers do NOT provide a DC output.

2. high voltage is equally dangerous regardless if referenced to ground or not - it is just that risks get somewhat shifted from one set of concerns to another which if done correctly can statistically appear safer. if isolation was always safer, utility company and national electrical code would not insist on grounding one terminal of the secondary.

3. are you familiar with breakdown voltage of LEDs? you do realize that they are still polarized devices and therefore work as rectifiers in AC circuit? what do you think will happen when capacitor gets charged?

1. I did mention that I was planning to use RECTIFIED 115VAC which will give me around (details removed by moderator)

2. I received a wicked shock once while changing a 24VDC lightbulb. Turned out the equipment had a floating ground and was supplied through a transformer. I measured 150 VAC to ground which is what I got walloped with.

So back to the original question re-worded a bit: Why is non-isolated and grounded 158VDC so much more dangerous than a grounded 115VAC or 220VAC? So much so that this forum does not allow circuit designs like this?

 

1. I did mention that I was planning to use RECTIFIED 115VAC which will give me around 158 VDC (if filtered/smoothed) to supply the 5 - 32VDC modules in series. The current limiting to be done with a capacitor or resistor.

And as I stated, if a capacitor is used, the rectifier has to be a full-wave bridge type and the capacitor has to be in series with the bridge input.

So back to the original question re-worded a bit: Why is non-isolated and grounded 158VDC so much more dangerous than a grounded 115VAC or 220VAC?

It's not.
It's just as dangerous.
The danger is for any line-powered device which is automatically grounded to earth ground, either AC or DC.

 

with isolation transformer you may touch one wire and feel nothing and consider it safe.
you may touch another wire and feel nothing and consider it safe.

then you may have a grand idea to touch both.... and get nasty surprise...

also in case of certain failure, secondary may be offset by primary voltage so you would see normal output across secondary but... voltage between either wire to GND will be significantly higher than that of secondary... this is why double insulation is used.

Main reason for using isolation transformer is to do galvanic isolation. This way your circuit is not referenced to ground and even you connect it to something else that lacks isolation, chances are there will be no sparks... Example, newbie using mains powered oscilloscope and trying to work on some dimmer circuit or antique TV or radio set...

Transformers do limit how much energy can be transferred when core is saturated but your safety would be compromised long before that point is reached.

 

(Details of a circuit we are not allowed to discuss removed my moderator)
If the wiring is done according to code as in any 220VAC appliance, will this work?
Is it wise? Is it safe?

I still do not understand how/why an isolation transformer makes this any safer than a typical grounded 220VAC appliance.
How is an isolation transformer any different than a "floating ground" on an AC system (very dangerous)?

 

No, it is not wise and it is not safe. That is why this forum does not permit the discussion of LEDs connected directly to the power mains. Use a transformer.

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