Informational How to consider the Working Voltage for splitting 2MOPP

Roland chung

Trusted Information Resource
#1
Hello folks,

There is a question about working voltage for the case that 2 MOPP is divided to 2 single MOPP (in former times: BI + SI). If working voltage of one concerned MOPP differ from the working voltage of the whole 2 MOPP, how to consider the working voltage for the latter MOPP?

For example: isolation barrier A between primary (240V) and secondary (48V) meets 1 MOPP @ 288V, barrier B from secondary to applied part in principle needs to fulfil 1 MOPP @ 288V. I think if primary to secondary does short, the voltage stressing the barrier B should be the mains voltage (240V), but not the summing voltage 288V.

Please kindly advise it.

Thanks and regards,
Roland
 
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Peter Selvey

Staff member
Super Moderator
#2
Sounds like an unusual situation.

From general principles of safety, we need to know how voltage reduces from 288V to 240V. The mechanism should be identified as part of the risk control, and verified as being reliable by inspection or testing. If it is reliable, it effectively becomes a means of protection, which then allows the next insulation to be assessed for 240V only.

As it is an unusual situation not really covered by the standard, it would be best to document in risk management as equivalent safety to 2 MOPP with working voltage of 288V.
 

Roland chung

Trusted Information Resource
#3
Hi, peter,

It is really not unusual, in particular for equipment which employs 1:1 transformer. The similar question is if 480V should be considered for the insulation next to the 1:1 transformer.
 

Peter Selvey

Staff member
Super Moderator
#4
Yes, the 1:1 transformer is not that unusual, but sufficiently unusual that the standard is not really written to address the situation.

In a 240V 1:1 transformer which is assumed to be 1 MOP (basic), it is possible to have 480V out in SFC. However, it is a very special fault that needs to occur in one place only.

Consider a transformer with input terminals P1 and P2, and output is S1 and S2, where P2 and S2 have the same polarity, and P1 is 0V (earthed), and the secondary is floating. The SFC analysis shows:

Short P1 to S1: S1 = 0V, S2 = 240V
Short P1 to S2: S1 = 240V, S1 = 0V
Short P2 to S1: S1 = 240V, S2 = 480V
Short P2 to S2: S1 = 0V, S2 = 240V

Another problem is the working voltage. Most secondaries are floating but still have a mid point earth by EMC caps, which sets up a working voltage somewhere between 0V and 150% (360V). The working voltage will vary at different points in the winding.

All of this sets up a very complicated analysis if we try to derive the minimum limits.

On the other hand, if the 1:1 transformer meets double for 480V at all points, we don't need to worry about any of this.
 

Roland chung

Trusted Information Resource
#5
Well said!

All of this sets up a very complicated analysis if we try to derive the minimum limits.
So, if I understand, the simplest way is to assume the highest voltage to which the insulation can be subjected, in this case 480V.

There is also a thinking. If the output S1 which mentioned in your post is tied to the earth directly, it is expected that 480V at S2 will not occur anymore. Earthing the S1 would be a means of protection.
 

Peter Selvey

Staff member
Super Moderator
#6
Yes, but there is a slight problem ... if the secondary is earthed, the isolation transformer stops being an isolation transformer, so it really has no significant safety function.

I think the real problem, as has been mentioned before, is that cr/cl limits in IEC 60601-1 (2nd ed, and 3rd ed for MOPP) are often overkill and include a lot of worst case assumptions. It is important to maintain double insulation from primary to secondary, but a limit of 16mm for 480V is too high for most situations. A distance of 9.6mm (IEC 60664-1, pollution degree 2, with interpolation) is more reasonable.
 

Roland chung

Trusted Information Resource
#7
Hello Peter,

Based on this topic, I want to ask another question which also related to transformer.

In my example, it is a surgical laser equipment. There is a secondary step-up transformer which used to charge the bulk capacitor to 700V Max. This voltage is actually the power source of laser. The step-up transformer is switching mode and the output is floating. My question is how to consider such transformer? If the output high voltage (700V) would go back to the input side (low voltage)?

Many thanks in advance.
 

Peter Selvey

Staff member
Super Moderator
#8
A theoretical floating output (internal to the enclosure) does not need to be considered for analysis for electric shock, because you would need two faults to get current to flow, and the first fault has no working voltage.

However, this would be difficult to realize, especially for higher switching frequencies (typically 40kHz or higher).

In practice, stray capacitance and/or EMC capacitors would be enough to give the output a real working voltage to earth or other circuits. Measurements would confirm this.

For a non-mains step up switching transformer there are a few options. One is double insulation. Another is fault tests (short and see what happens). Another could be surrounding the circuits with earthed parts. There has to be a plausible way for the voltage to get outside the enclosure to be dangerous. Sometimes the construction is such that there is no realistic way, even though there is 700V floating around.
 

Roland chung

Trusted Information Resource
#9
Peter, I appreciate your input.

In your opinion, one approach to evaluate the step up transformer is to meet requirements of double insulation. But I don'know why. Actually, such transformer is always not isolating.

Now, the R&D want to earth one side of the high voltage and this would cause 700V reference to earth. From my side, I don't see a problem since the enclosure is metallic and being earthed. If the other side shorted to enclosure, that would be no different to short circuit the output of the step up transformer.

Could I have your opinion again?
 

Peter Selvey

Staff member
Super Moderator
#10
Double (reinforced) insulation simply means you don't have to think about faults. It is not required, just one option.

The fault approach also works well for low power HV secondary circuits (e.g. transducer drives in ultrasounds). Usually the current limit is <250mA which means almost all faults will pull down the HV rail. You would need a really special fault which keeps the rail >60Vdc and can allow the voltage to get outside the enclosure. It can happen, if the designers stupidly put the ethernet/LAN circuits right next to the HV circuits. But normally circuits connected to the outside world are well removed from internal HV circuits.

However, in the case of a high power HV circuit, some care is required, because we can no longer rely on the current limit to pull down the rail in fault condition. This means faults can propagate further and less predictably. In that case, it's better to isolate the HV rail itself (parts with low impedance to the HV rail).

Direct grounding of the 0V side is OK, provided the impedance is low enough to trip the overcurrent protection in fault condition.

Usually faults directly from pri-sec of a transformer will destroy the switching circuits, killing the HV output so double insulation is not required. But a test would be necessary for verification.
 
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