Author Topic: How close can a 240 VAC trace to its neutral line  (Read 611 times)

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Offline drakejest

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How close can a 240 VAC trace to its neutral line
« on: October 27, 2020, 05:30:01 pm »
I am designing for a 240VAC 3phase ADC Front end. I have decided to design saftey clearance for surges of up to 2000 volts spacing wise. But  as i am planning the layout i am running to trouble with the neutral trace having to come close to each phase. Kind of tricky since the adc takes in a differential so each phase must have a accompanying neutral.

So how close can i trace a neutral line to the each phases of the 240VAC line?

I also of thought of placing some components of the bottom side of the board, if i trace all neutral on the bottom would the 1.6mm board thickness alright?

Image

The pads labeled with R,F are the connector, the three connector on the left side are for the 3 phases while the one spaced on the right is for the neutral. The component on the bottom layer (blue) are local tvs
 

Offline m k

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Re: How close can a 240 VAC trace to its neutral line
« Reply #1 on: October 27, 2020, 06:56:42 pm »
I think the rule goes something like max peaks and then small multiplier.

I wouldn't accept 2mm.
 

Offline TERRA Operative

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Re: How close can a 240 VAC trace to its neutral line
« Reply #2 on: October 28, 2020, 01:19:59 am »
Can you use isolation slots?
Any circuit design must contain at least one part which is obsolete, two parts which are unobtainable, and three parts which are still under development.

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Offline Doctorandus_P

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Re: How close can a 240 VAC trace to its neutral line
« Reply #3 on: October 28, 2020, 01:14:44 pm »
Voltage differential between GND and a phase is about 70% or so of the voltages between the phases themselves, so you will need to keep very similar clearances.

1.6mm fiberglass and epoxy would be plenty isolation between GND and the phases. But be careful with capacitive coupling and vias. You can define ONE potential as the gnd level for your circuit, It can even be one of the phase signals, but it would be much more logical to use the Neutral as your GND.

In total the high voltage area should be pretty small. Just from the connector to some resistor divider, and maybe some capacitors for frequency compensation.

It is perfectly all right to use the Neutral of the 240Vac as "GND" potential of your ADC, as long as the power supply is isolated and has sufficient isolation. This way you have no issues with linearity of complex analog circuit in the frontend of your ADC. You can also connect a microntroller or whatever other circuitry needed directly to mains. But then you do need an isolation barrier in the digital domain on the other side of the ADC. It's also a complication for reprogramming the uC during development.
 

Offline drakejest

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Re: How close can a 240 VAC trace to its neutral line
« Reply #4 on: October 28, 2020, 02:28:32 pm »
It is perfectly all right to use the Neutral of the 240Vac as "GND" potential of your ADC, as long as the power supply is isolated and has sufficient isolation. This way you have no issues with linearity of complex analog circuit in the frontend of your ADC. You can also connect a microntroller or whatever other circuitry needed directly to mains. But then you do need an isolation barrier in the digital domain on the other side of the ADC. It's also a complication for reprogramming the uC during development.

The adc is comepletly isolated power and signals by digital isolators, and have been designing that the citical compenents are capable of tolerating upto 2kV surges. And yes trying too keep even spacing and reduce the High voltage. Im worried that a phase (or multiple phase) will just just arc to the netural line, from the connector going straight through the pcb directly to the bottom neutral traces
 


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