GDT between Live and Neutral in offline SMPS

[cupoftea]

Hi,
In an offline 240VAC SMPS, what would be the use of putting a 470V GDT between Neutral and Earth?
The GDT being Bourns 2027-47-SM

Presumably its in case Local Earth gets lifted up in potentail by a lightning strike? But this GDT is so small I cant see it being of use?

Bourns 2027-47-SM

https://www.bourns.com/docs/Product-Datasheets/2027-xx-SM.pdf

 

[betwixt]

More likely in case the neutral line spikes higher than Earth otherwise it would need an independent ground wire to physical ground beneath your feet. I would guess it is to limit spikes that might otherwise break down an insulation barrier at the PSU input somewhere. Don't underestimate the power they can absorb. Consider they are intended to clamp very brief pulses and are in series with the inductance of the power feed cable so even a few mA would be effective, they can typically handle a few Amps for uS or mS. I would be surprised if there was only one between N and E, a second one between L and E would seem a sensible addition.

Brian.

 

[Easy peasy]

a lightning strike can lift both Ph & N above earth - you did not say whether there was one on the Ph too (?)

 

[FvM]

Gas discharge tubes without additional current limiting elements are not suitable for AC power line surge protection.

 

[andre_luis]

In addition, spikes from these discharge tubes are strong sources of EMI.

 

[cupoftea]

a lightning strike can lift both Ph & N above earth - you did not say whether there was one on the Ph too (?)

Thanks, but no, its only from neutral to earth.
As far as i can see, its only lighting strikes that this GDT could be useful against? (assuming it had the suitable current limiting element in there)

Gas discharge tubes without additional current limiting elements are not suitable for AC power line surge protection.

Thanks. There is only a 12R series resistor and its in the live line for current limiting. What sort of current limiter were you indicating may i ask? The common mode choke woudl also provide impedance as you know, though i suspect even being a CM choke, it would likely saturate at these current levels.

Also, i presume that the GDT would likely fail open, and so adding this GDT, even without a current limiter, wouldnt do any harm?...just perhaps a waste of money.

Why is is not common to see GDT proection to earth on offline SMPS's?

Also, as you know, a 500V AC test is often done from live and neutral connections to earth in production.,.......so a 470V GDT would surely break over during this.....so why do people use 470V ones?

 

[betwixt]

I worked on a project for detecting static charge on large carts while they were pushed around a factory. As with GDTs being high impedance until striking voltage, I used neon lamps and wound a pick-up coil wound around them. The ionization as they flashed was picked up by the coil and used to drive a counter. If nothing else, proving they certainly radiate EMI!

I don't recall ever seeing GDT on a PSU though.

Brian.

 

[FvM]

Please review this previous thread about GDT behavior and related questions: https://www.edaboard.com/threads/ga...sient-protector-upstream-of-mains-fuse.372840

I doubt that you find substantiated information about open versus short failure of GDT when loaded beyond its rated energy. I believe that open circuit might happen if the GDT case explodes but also short circuit if the electrodes melt.

As stated before by others, small GDT are not designed to be used for AC mains voltage and not used in professional designs.

 

[Easy peasy]

GDT's are often used in series with MOVs, and sometimes in series with caps and limiting resistors - they have a faster response time than MOV's which is useful in some apps

 

[Easy peasy]

here we go ....

 

[cupoftea]

Thanks all and Thanks to Easy Peasy for the images. Symetircal from live and neutral to earth.
I am actually wondering if having just one GDT from neutral to earth is about giving some protection for when the Neutral gets disconnected...because as you know, normally neutral and earth are connected together right at the installation/property etc (TN-C-S)