Issues with Dielectric Strength Testing - IEC 60601

S

Sclpt

#1
Hello All,

This is my first time involved with 60601 testing so please forgive any ignorance.

I am working on a robotic system for orthopedic surgery currently undergoing 3rd edition 60601 (the system was previously certified to 2nd edition). Our test house is performing clause 8.8.3 by applying 1500V directly to our UPS (uninterruptable power supply) with the UPS breaking down between input and earth at around 500V.

The power flow in the system is as follows: power from the mains, to an isolation transformer (3rd edition certified) to the UPS (60950 and 62040 certified) to various components in the system (PC, monitor etc.), most importantly the cutter console (unit which powers the drill, which is 60601 3rd edition certified and has passed 8.8.3) to finally the drill which is the Type-B applied part and which contacts the patient. All components are within a metal and polyurethane enclosure under sterile drapes.

Is our test house testing this clause correctly?
Does the UPS classify as solid electrical insulation and insulation that is relied upon as means of protection?

I appreciate all insight, the test engineer is not being particularly helpful in helping me understand this issue.

Thank you
S
 
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Marc

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#3
Another quick "Bump". My Thanks in advance to anyone who can help with this one.
 
R

raymond02302

#4
Technically, 500 V can not puncture the CR/Cl between earth and input of UPS which is 950 certified. I guess the test house did not disassemble the capacitor or SPD between input and earth that will lead to increased trip current. Test house may mistake this condition for failed test.
 
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N

NickM

#5
Couple things:

1) Have they done the hipot at AC or DC? If they are doing it at AC, ask if they would accept doing it at DC. Most will as long as the DC value is square root of 2 times the AC value (1500x1.414=2121VDC)

2) If they insist on AC, ask what the failure progression looked like in terms of leakage current on the hipot tester and what the ramp speed and trip current limit they set.

2a) If the leakage current slowly increased as the voltage increased; that is a typically a sign of a capacitor being charged or some auxillary leakage in the produt and not typically a sign of insulation breakdown. If this was the characteristic, then I would make sure they have a hipot tester that can go to 100mA (only available on older units - newer units have a limit of 10 or 20 mA) if they insist on doing AC hipot.

2b) If the leakage current stayed very low and then all of the sudden tripped the tester; that is a sign of either an insulation breakdown (failure) or some type of suppression device (i.e., TVS) doing its job. These devices (including EMI filters) can be disconnected during the test.

3) If you are still having trouble passing, then it is time to start hipot only portions of the circuit at a time to isolate where the failure is coming from.

My experience has been that most hipot failures are not really failures. Often it is either due to the test setup (i.e., putting leads on a conductive floor or too close to each other or an earthed part) or due to not understanding the circuit you are testing.

Hope this helps.

Regards,
Nick
 
S

Sclpt

#6
Thanks so much for the responses so far,

1.Currently they have been doing the hipot at AC voltage, I will enquire regarding DC.

2. I witnessed the failure, essentially the second he started increasing the voltage the current increased, there was no sudden tripping, it was a constant progression.

I have recieved this response from the test engineer regarding the UPS failure, but not entirely sure what to make of it.

[FONT=&quot]If you remove the VDR and GDT (in the UPS), you effectively have a layer of solid insulation between the L/N terminals of the UPS and earth (i.e. pcb material) and this is subject to the requirements of 8.8.3 for the dielectric strength test, but you are left with the problem of addressing the components bridging that insulation. [/FONT]
[FONT=&quot]8.5.1.3 states that components that are connected in parallel with an insulation, an air clearance, or a creepage distance must comply with 4.8 and 8.10.1 if they are to be regarded as a means of protection.[/FONT]
[FONT=&quot]4.8 states that all components, the failure of which could result in a hazardous situation shall be used in accordance with their specified ratings unless a specific exception is made in this standard or through the risk management process. The reliability of components that are used as means of protection shall be assessed for the conditions of use in the equipment. You do not specifically need to pass the 1500V electric strength test, but you do need to provide conclusive evidence that it meets the requirements for a means of protection under the conditions of the application.[/FONT]
[FONT=&quot]Given the requirements of EN 60950-1 and the failure of the GDT / VDR combination to meet them, I would conclude that they are not reliable for the purpose of providing a means of protection.[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]If you can’t provide us with a sample that provides evidence of the necessary EN 60950-1:2006 compliance and the manufacturer is unable/unwilling to help resolve, I do have one final line of attack which might provide a solution. As the UPS is supplied through an isolation transformer, it should not be possible for any significant leakage current to flow to earth, so it may not be a problem if the chassis becomes connected to the input of the UPS, even if the earth is disconnected, provided that there is no other accessible point on the equipment to which a leakage current could flow. I don’t think there is, but I would need to convince myself of this. [/FONT]
[FONT=&quot] [/FONT]
 
R

raymond02302

#7
Just Increased current in an uncontrolled manner(sudden increase) constitutes a failure. An constant progression does not constitute an failure, which is mainly because of leakage of capacitor or bleeding ressitors and SPD etc.
Capacitor complied with IEC 60384-14 (see CL 8.8.3), bleeding resistor and SPD (see IEC 60950) can by removed during Hi-pot test.
Combination of VDR and GDT is allowed used between mains input and earth, provided the GDT complies with requiremnts of CR,CL and hipot test for basic insulation (see 108/INF/471 by IECEE)
VDR alone also can be used between earth and main input for three cases,
- type B plugable equipment
- permanently mounted equipment
- equipemnt with extra earthing terminal
 
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S

Sclpt

#8
Thank you Raymond,

The test engineer has now tested at DC and at 100mA as suggested above and both failed as before, constant progression of current with voltage. According to him the GDT fails the hipot test for basic insulation (we have tested two samples).

Our equipment is type A pluggable so the VDR can't be used alone.
 
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