[Amps] Insulation tester

[Larry Benko]

  Don,

I use an ignition transformer for a fuel oil furnace to test breakdown 
voltages.  These are 120VAC primary and a grounded center tapped 
secondary of 6kVAC to 12kVAC.  They are readily available, are impedance 
protected and made by companies such as LaFrance and Dongan.  The model 
I have is a Dongan LJH-90A and it has a secondary of 5000VAC-GND-5000VAC 
(14kV peak across the entire secondary) and the maximum current it 
produces is 23mA AC rms.  The output can be shorted indefinitely (which 
could happen if the igniter in the furnace was clogged).  I use a 3A 
variac to power the primary and use some  Caddock MX440 5meg ohm 
resistors in series with the secondary.  These resistors are good for 
11kV across them and limit the current to sub-milliamp levels.

I generally limit the current via the resistors and when testing and a 
breakdown doesn't cause damage to switch contacts, relay contacts, air 
variable capacitors etc.  If possible I only use one side of the 
secondary and place a multimeter in series with the center tapped lead 
(multimeter at ground potential) and can watch the  leakage that occurs 
prior to a breakdown.

Concerning the breakdown voltage at HF vs 60Hz I have a pdf document 
showing a fairly good correlation between testing at HF and at 60Hz.  If 
you want the document I can send it to you.

73,
Larry, W0QE

On 11/25/2010 7:49 PM, donroden at hiwaay.net wrote:
> Has anyone used neon transformers to build an insulation tester  ( Megger  ).
>
> I need something to go to around 10KV for testing capacitors ( type 291 )
> under a high voltage /low current condition.
>
> Don W4DNR
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