Surprising Voltages On Hi Fi Equipment Cases !

HearNoEvil

Standard Member
Hi, I am wondering if anyone else has experience of this.
I recently noticed my audio equipment have a voltage on their cases and Phono Inputs !.
The seperates are all Double Insulated. I used a Fluke 89IV multimeter and was surprised to find voltages ranging from 38Volts to 195Volts on the exterior of the metal cases, with regards to Earth, more surprisingly with no components connected, also at the Phono Inputs/Outputs (Screen and Centre). :eek:
These are not Ghost Voltages as the meter discerns and disregards these.
The voltages also carry a small but measureable current, ranging from 38microAmps to 98microAmps with regard to earth. I have checked a range of two wire, double insulated audio equipment and it seems they are all the same, regardless of make.
I am wondering if anybody else has noticed this, or can explain it.
The voltage is only present on two wire, double insulated equipment.
It is obviously not present on 3 wire, earthed equipment.
If your double insulated equipment is connected to earthed equipment you will not find this. With components isolated from each other it is surprising.
I would appreciate anyone's houghts on this, Many thanks.
 

alexs2

Distinguished Member
Its apparently not unusual(in that our engineers here test all electrical equipment,new and old,for leakage currents etc),but the product of even 200V @ 100 microamps is about 20mW,which is unlikely to cause any harm at all.

It does however demonstrate that if the double insulation was compromised in any way,there could be trouble to put it mildly.

IEC document 60601 covers most of this,and allows for up to 1-3mA leakage in most circumstances,although it also specifies a measuring device that takes into account the specific impedance offered by the human body,rather than a multimeter alone,which will not take that into account.
 

HearNoEvil

Standard Member
Thankyou very much Alexs2 for the reply. Very informative and interesting. Many Thanks.
I would never have dreamed the case of my audio gear would carry any voltage or current.
It is reassuring to know it is not dangerous, but I can't help wondering if it is dangerous to the connecting equipment ! as these voltages are present at the line level inputs/outputs.
I have a concern with a NAD C660 CD Recorder especially.
I have metered 137 Volts with 185microAmps present on its case, with regards to Earth. Although this voltage/current is not detectable to the touch, it is feeding into the input of a NAD C372 Integrated Amp. The Amp itself, with nothing connected, puts out 194Volts at 98microAmps !
I have real concerns with this.
Would you consider having the items tested/repaired or is this totally normal ?
I understand the transformers etc. could be the major contributors to this by Field Effects causing Eddy Currents in the casing. Could this be true ?
Your observations on Double Insulated equipment's safety is very interesting, something I had not considered until now.
Again, many thanks for your time and comments on this matter, All The Best
 

alexs2

Distinguished Member
I'm certainly no expert on this topic,but do bear in mind that you're measuring a potential difference and current via a multimeter,which as I said previously does NOT take into account the impedance of the human body to an alternating current,which in itself is quite substantial,and assuming you arent standing in a bucket of water(!)may run to several MegOhms in ideal conditions,to several hundred ohms when standing in a bucket of conductive liquid(!).

The measuring device specified for medical equipment has an input impedance of not less than 1 MegOhm,and a flat frequency response from DC to 1MHz,and is used to test equipment to the highest current standard.

By measuring the current with a multimeter,which is measuring the leakage current at effectively zero resistance,the results will be different.

In any case,since you evidently have concerns on the issue,the safest place to start is with the manufacturer,and work from there
 

HearNoEvil

Standard Member
Once again thankyou alexs2 for your response.

I have been able to research a little more into this situation from your reply.
It seems that all double insulated equipment ( items with 2 prong power cables, no earth ) suffer from what is known as "touch/chassis leakage".

This is a result of the metal casing being "grounded" to the common ground of the circuitry, in an effort to provide some shielding from RF getting into or out of the equipment. There can be no mains decoupling filters which connect to the chassis, so at least the chance of a shock from this cause is eliminated or at least "minimised ", but if something catastrophic were to go wrong it leaves the potential for mains to appear on the casing and all signal inputs and outputs (RCA, Phono etc).

The voltages / currents arise due to capacitive coupling between the primary and secondary sides of the power transformer. Due to the fact that with double insulated items there is no "earth" available to use as a shield between input and output windings, this results with current being passed to the common ground of the unit's circuitry, which as I previously mentioned is connected to the case.
The more power something uses results in more power being capacitively coupled to the chassis ground and case. Switch mode powers supplies would not be exempt from this either, although they have smaller transformers, less capacitance, they run at comparibly higher frequencies and results in high amounts of capacitively coupled power being transferred.
Although this is normal, in double insulated devices, it appears there is no substitute for proper Earthing via a 3 pin plug design.

Please note this effect is also compounded when more double insulated items are connected together via interconnects ! :eek:
 

emmujo

Novice Member
I had the same problem of mild shock and solved the problem by connecting a small wire to the chasis and the earthing in the wall plug.
 

The latest video from AVForums

Are the TCL MiniLED TVs better than OLED? TCL Interview with Marek Maciejewski | AVForums Podcast
Subscribe to our YouTube channel
Support AVForums with Patreon

Top Bottom