3-pin (Gas Discharge Tube) Surge arrester rating DC spark over voltage

[Sherlock_H]

3-pin (Gas Discharge Tube) Surge arrester rating "DC spark over voltage"

Could someone please help with a question about Gas Discharge tube, surge arresters rating?

I am looking at a public address audio equipment which uses a 100vrms audio lines driven from a audio transformer. Currently the equipment has a single two pin epcos surge arrester placed across audio line, (It's blue and marked 500). This has a 'DC spark-over' voltage of 500V and discharge class of 20KA/20A.

I have been asked to change this so its protects down to Earth rather than just across the line. Because of a lack of space I have chosen to use an equivalent 3pin type, with the centre pin being the Earth connection.

I have two problems that are confusing me,

1. I am not sure whether the 'DC spark-over' rating of 3pin arrester is still rated between the end pins down to the centre pin or still across the end pins. So, is a 250V 3pin type is equivalent to a 500V two pin type?

2. Assuming the voltage between each line to Earth will be half the voltage that's between the audio lines, do I choose an arrester that's also half the voltage across the audio lines, i.e 250V instead of 500V (depending on the answer to first question).

Thanks.

 

[berkeman]

Could someone please help with a question about Gas Discharge tube, surge arresters rating?

I am looking at a public address audio equipment which uses a 100vrms audio lines driven from a audio transformer. Currently the equipment has a single two pin epcos surge arrester placed across audio line, (It's blue and marked 500). This has a 'DC spark-over' voltage of 500V and discharge class of 20KA/20A.

I have been asked to change this so its protects down to Earth rather than just across the line. Because of a lack of space I have chosen to use an equivalent 3pin type, with the centre pin being the Earth connection.

I have two problems that are confusing me,

1. I am not sure whether the 'DC spark-over' rating of 3pin arrester is still rated between the end pins down to the centre pin or still across the end pins. So, is a 250V 3pin type is equivalent to a 500V two pin type?

2. Assuming the voltage between each line to Earth will be half the voltage that's between the audio lines, do I choose an arrester that's also half the voltage across the audio lines, i.e 250V instead of 500V (depending on the answer to first question).

Thanks.

Could you post links to the datasheets for the old and new arrestors? That would help us answer your questions. Thanks.

 

[Sherlock_H]

Thanks for the reply,
Apologies for the delay.

I believe this is the spec for the existing GDT, which is across the 100V audio lines.
http://www.epcos.com/inf/100/ds/a81a500xx1490.pdf

Reading on the epcos website it seems to me that the 3-pin types are more like

two separate GDT connected in series with the centre electrode as the common..
(Reference the link below)
"Test configuration for 3-electrode arresters:
The specified spark-over voltages, insulating resistance and capacitance refer to the respective measurements between one of the two wire electrodes (a/b) and the center
electrode (c).
Unless otherwise specified, the impulse or AC current is applied simultaneously from the two line electrodes to the center electrode with the defined value as the total current through the center electrode (c). "

http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/ProtectionDevices/SurgeArresters/PDF__Arresters/PDFDefinitions,property=Data__en.pdf;/PDFDefinitions.pdf)

I'm still not sure what voltage to choose.

 

[berkeman]

Thanks for the reply,
Apologies for the delay.

I believe this is the spec for the existing GDT, which is across the 100V audio lines.
http://www.epcos.com/inf/100/ds/a81a500xx1490.pdf

Reading on the epcos website it seems to me that the 3-pin types are more like

two separate GDT connected in series with the centre electrode as the common..
(Reference the link below)
"Test configuration for 3-electrode arresters:
The specified spark-over voltages, insulating resistance and capacitance refer to the respective measurements between one of the two wire electrodes (a/b) and the center
electrode (c).
Unless otherwise specified, the impulse or AC current is applied simultaneously from the two line electrodes to the center electrode with the defined value as the total current through the center electrode (c). "

http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/ProtectionDevices/SurgeArresters/PDF__Arresters/PDFDefinitions,property=Data__en.pdf;/PDFDefinitions.pdf)

I'm still not sure what voltage to choose.

From your 2nd link, it does sound like the arc-over voltage for the 3-pin devices is specified as each of the two fire voltages from the two electrodes (a/b) to the center electrode (c). So if you want a differential a-b fire voltage of 500V, you would use a 3-pin arrestor rated at 250V.

You may want to give their Customer Support folks a call to verify this, or if you can order a small quantity, just get some ane measure them yourself.