Monday, October 26, 2020

Using a Class I power supply in a Class II construction

I have to admit that I was a little skeptical when I was first told it was possible to use a Class I power supply (with an earth ground connection) in a Class II (no earth ground connection) construction.  After some discussion with a colleague, he pointed out that if certain actions are taken, it is perfectly safe - even in a medical application.

A general statement of safety is that a product must have two levels of protection between live (primary) parts and the end user.  If one level of protection fails, the end user is still safe.

A Class I example would be a clothes dryer. Usually they have an earth ground connection which is securely connected to the drier’s metal enclosure.  This is to protect the user if the insulation on the AC input cable frays or the heater element malfunctions and makes contact with the enclosure.  The fault current would flow through the earth wire, tripping the home’s earth leakage breaker and the AC power would be disconnected.

For our clothes dryer example, one protection level is the insulation on the internal wiring, the second level is the protective earth connection.

As mentioned in previous blog articles, the home healthcare standard (IEC-60601-1-11) stipulates the need for Class II equipment, as the integrity of the safety earth connection cannot be guaranteed in the home environment.

TDK-Lambda’s medically certified open frame CUS600M series (see Figure 1) can be used in either Class I or Class II applications.  It has two levels of protection, achieved through the use of double or reinforced insulation.



Figure 1: CUS600M power supply

 

The CUS600M/EF versions though have a metal enclosure and fan fitted. See Figure 2.  This changes the situation for a Class II application as during a single fault condition, the metal cover may  become live. If the cover was accessible to the user, we would no longer have two levels of protection. Fortunately, there is a solution!

Figure 2: CUS600M/EF power supply

 

First, the power supply must have 1 x MOPP (Means of Patient Protection) from the input to ground (chassis) and 1 x MOPP from the output to ground. 


Our CUS600M/EF has that - see Figure 3.


Figure 3: CUS600M/EF MOPP isolation barriers

Second, ensure the output touch current does not violate the safety requirements of the end equipment.


In our case the CUS600M touch current (enclosure leakage) is very low, <100µA, which should be acceptable in most applications.

Third, the power supply chassis should have 1 x MOPP isolation from the final equipment enclosure.

From Figure 4 it can be seen that we now have achieved our two levels of protection, even without the use of an earth ground connection.



Figure 4: CUS600M/EF isolation from the final enclosure

The 1 x MOPP isolation between the power supply and the system enclosure can be achieved by mounting the power supply on a plastic / non-conductive insulating plate or by mounting the unit on plastic / non-conductive stand-offs. See Figure 5.


Figure 5: Power supply with a metal chassis mounted on plastic / non-conductive stand-offs

Last, the EMC performance must not be adversely affected


The EMI performance of the CUS600M open frame power supply in a Class II application already meets EN55032-B for both conducted and radiated emissions.  The same applies to its immunity performance.

 

Even a Class I modular power supply like QM series can be used in this manner too, as the output to chassis isolation is 1 x MOPP. It should be noted that many other modular power supplies just have basic (non MOPP) isolation.

For further advice and application support on this, or any other power supply related topic, please contact your local TDK-Lambda office.


Power Guy

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