Accessible parts per 8.4.2 b) - Touch currents in combination with MOOP or MOPP?

Benjamin Weber

Trusted Information Resource
#1
Dear all,

I have a recurring discussion regarding whether MOOP or MOPP are required for accesible parts per 8.4.2 b).

Some people say: These parts have a chance to be connected to the patient indirectly via the operator, hence MOPP have to be applied (but still with touch current limits).

In my understanding that is wrong: MOPP have to applied only to applied parts or to parts that have to be treated as such (per cl. 4.6). But those parts fall under cl. 8.4.2 a) (patient leakage and auxiliary currents).

What do you think, is the follwoing a correct summary?
  1. Applied part or treated as such:
    -> Cl. 8.4.2 a) (Patient leakage, auxiliary currents) and cl. 8.5.1.2 (MOPP) apply

  2. Accessible part with direct/indirect patient connection via operator possible:
    -> Cl. 8.4.2 b) (Touch currents) and cl. 8.5.1.3 (MOOP) apply

  3. Accessible part only to the operator:
    -> Above 42,4Vp / 60Vdc and E>240VA*60s or stored E>[email protected]>2V: Cl. 8.4.2 b) (Touch currents) and cl. 8.5.1.3 (MOOP) apply
    -> Else: Cl. 8.4.2 c) (No MOP) applies
 
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Benjamin Weber

Trusted Information Resource
#2
Hm, so far more than 60 views, but no answer or comment.

I wonder if the answer is either so obvious for others or really no one has on opinion on this?
 

Peter Selvey

Staff member
Super Moderator
#4
I agree that parts with MOOP that the patient might tough (directly or indirectly) do not in principle need to meet MOPP requirements. MOPP applies only to parts that must touch for the intended purpose.

Clause 4.6 addresses the issue of incidental contact. But in practice it's rarely an issue as the parts with high probability incidental contact are usually contiguous with the applied part. For example, ECG cables could incidentally contact the patient. But the wire inside the cable is patient circuit anyway. So instead of arguing MOOP or MOPP just call it MOOT ;)

Although someone might argue that incidental contact is important, in fact it's not. IEC 60601-1 has higher limits for MOPP vs MOOP due to the various factors in the nature of contact such as skin impedance bypassed, patient unconscious or immobilised, earthed, long term contact, and small contact areas (current density). When assessing 4.6, manufacturers can assess the type of incidental contact and see if it ticks these kind of boxes and hence raises the level of concern. If yes, apply MOPP limits. If not, MOOP is fine.

In fact a lot of applied parts don't even tick these boxes, but for the purpose of standardisation (keeping it simple) it makes sense to use MOPP for all applied parts. Or, maybe they could extend 4.6 to say that in RM, if the nature of contact is benign, MOOP could also be used for applied parts?
 
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