MOP (Means Of Protection) - Creepage and Clearance questions

[fonk140]

Hello I have some questions concerning the IEC 60601-1 standard, I would be very grateful if you could clarify me these points:

- In our device 3 wires go from the power ac inlet to the power supply, how 2 MOOP are achieved between the two phases?, and between each phase and protective earth?. It is enough with the normal insulation of the wires or something more is required?, creepage and clearance distance apply in this case?. Does the fastening of the wires need special protection?

- Are creepage and clearance distances required in secondary circuits with voltages below 15 V?

-Does plastic enclosure counts as 1 MOOP or as 2 MOOP?

- Can/Should secondary circuits and SIP/SOP circuits be earthed to protective earth?

Thanks in advance!

 

[Peter Selvey]

Re: MOP (means of protection), Creepage and Clearance questions

The wires from the wall to the equipment are covered by separate standards and regulations, usually the designer of the equipment just buys approved cords/plugs etc.

If the wire is non-detachable (wired into the equipment to terminals), there are a bunch of requirements in the standard to make it safe. Clause 7.3 covers marking, 8.6.2 for the earthing terminal, 8.11.3.5 for cord anchorage, 8.11.4 for general requirements for terminals. There may be more. It is an important area so better to involve someone with some reasonable electrical background in the design, and best is to then get it independently checked out by a qualified test lab to catch any residual mistakes.

Generally 15Vdc circuits do not need any special isolation unless there is a potential for direct connection to the patient.

SIP/SOPs circuits can be earthed or floating. For SOP/SOPs, there is some complicated stuff in the standard but in the end there is usually nothing special in the end and it would take a lot to explain.

 

[fonk140]

Re: MOP (means of protection), Creepage and Clearance questions

Thank you very much for your answer Peter.

The questions about the wires were about the internal wires of the equipment that connect internally the mains voltage to the internal power supply. Do they need any special insulation or protection?

Does creepage and clearance apply only to traces in PCB or also to "normal" wires and cables?

Regarding the plastic enclosure, how many level of protection it provides, 1 or 2 MOP?

 

[Peter Selvey]

Re: MOP (means of protection), Creepage and Clearance questions

The best way to think about it is to consider the mains voltages as being "hot" and the "means of protection" is then the barrier around them to prevent people burning themselves.

For electric shock, the standard does not care about design or spacings inside the mains parts, only the quality of the barrier around them.

For simplicity, most designers segregate circuits so the barrier is nice and clear and easy to verify for compliance. On PCBs, sometimes designers will put a dotted line on the silkscreen to show where the barrier is. Usually only a few component bridge the barrier, like optocouplers, capacitors, transformers, and these are specially designed to provide a safety barrier. Outside of PCBs, wiring insulation or physical separation is the most common method to form a barrier.

A common mistake is to use an certified open frame power supply but then run secondary wiring touching or close to bare mains parts such as a primary heatsink. The only barrier then is insulation on the secondary wiring, which is usually not enough for 2 MOP. Common solutions are for example routing the secondary wiring away from the power supply, or putting sleeving the secondary wiring.

Generally, the external enclosure is well removed from the mains circuits, with the air gap to the enclosure is >> 2 MOP (typically 5mm is enough for 2 MOP). If so there are no electrical tests. But it still may form a mechanical protection which is subject to non-electrical tests, such as force tests, impact, mold stress etc.

 

[fonk140]

Re: MOP (means of protection), Creepage and Clearance questions

And how is calculated the protection level of an insulated wire?

For example if we have a 18 AWG wire with a nominal tension of 300V can we determine that it has 2 MOOP at 220V or it necessary to do a an HIPOT test?

 

[Peter Selvey]

Re: MOP (means of protection), Creepage and Clearance questions

If the wiring is rated and certified for 300V this can be treated as 1 MOP without further test.

Strictly the 300V rating should be using IEC standards, but in practice most agencies (and manufacturers) will also accept US based tests (e.g. UL rating for 300V).

 

[fonk140]

Re: MOP (means of protection), Creepage and Clearance questions

And what voltage should be certified to have 2 MOP?

 

[fonk140]

Re: MOP (means of protection), Creepage and Clearance questions

And what about the fastening of the wires, how can we determine if it provides 1 or 2 MOPs?

 

[Peter Selvey]

Re: MOP (means of protection), Creepage and Clearance questions

You need to check the rating (spec sheet) of the wiring. It should show if the wiring is designed to provide double insulation (2 MOP). It is fairly common so should not be hard to find. The rating may be 600V, but check also that it is certified as being double insulation.

If a detached wire can cause double insulation to be bridged (e.g. mains wire disconnects and can touch secondary, or vice versa), it should have two means of securement. Examples of common methods are double crimp connectors (connector with wire and insulation separately crimped), or solder + cable tie.

 

[fonk140]

Re: MOP (means of protection), Creepage and Clearance questions

Two more questions:

1) if a secondary cable could go near or even touch a primary (mains) wire, it is necessary that the secondary cable has a nominal working voltage equal or superior to mains voltage to comply with 2 MOP between primary and secondary?

2) you said in your first post that secondary circuits doesn't need isolation, so in what situations are creepage and clearance of table 15 (distances for secondary circuits) used?