Does it matter how an EMC/EMI filter is wired to a power supply?

An email was recently received by TDK-Lambda’s Technical Support from a technician who was fault-finding a video system.  His questions about how an EMC filter should be connected were relevant and warranted a blog article on the subject!

As indicated in Figure 1, the EMC filter is situated between the AC input to the system and the power supply providing DC voltage(s) to the system load.  The filter’s function is to reduce incoming noise from the AC input and/or outgoing noise to the AC input from the power supply.

Figure 1: System block diagram

Regarding connection to the filter, let us now look at a typical EMC filter - the 250Vac 10A rated RSEN-2010 for example (see Figure 2 and 3).

Figure 2: RSEN-2010 filter

Figure 3: RSEN schematic

Reviewing Figure 2, the nomenclature “LINE/LOAD” on the left hand and right hand side of the label indicates that either set of the terminals can be used for the input or load connection.  It can be also noted that there is no indication of where the Line or Neutral should be wired to, just numbers 1, 2, 3 & 4.  Which connections are made to these terminals is very important.

On the right hand side of the label and schematic (Figure 3), two additional capacitors, known as “Y” capacitors can be seen.  These provide a low impedance path to earth ground to reduce high frequency common-mode noise.   When the filter is used to reduce system noise from the power supply reaching the AC source, terminals 3 and 4 should be connected to the power supply.  If the filter is being used to reduce externally generated high frequency noise from entering the system then terminals 3 and 4 should be connected to the AC source input.

It is essential that if the AC line is connected to terminal “1”, then terminal “4” should be connected to the power supply Line terminal.  Likewise with terminals “2” and “3” for the neutral wiring as shown in Figure 4.

Figure 4: Connection of the filter to the AC and the power supply

If the wiring to “3” and “4” is crossed, there could be safety issues for a power supply with a single input fuse situated internally in series with the AC Line terminal.  

If the AC is correctly wired (Figure 5a), in the event the internal power supply fuse was to open due to a fault, the internal circuitry is isolated.  It is important to note that at the breaker panel, where the AC first comes into the building, the neutral input is grounded to earth.

Figure 5a: Correct AC connection to a single fuse power supply

If the Line and Neutral is reversed to the power supply due to incorrect wiring (Figure 5b), in the event of the fuse opening the internal circuit is still live.  Two issues can arise with this scenario.  A service technician fault-finding the system could receive an electrical shock, particularly with an open frame power supply without a cover.  Secondly, if an internal short was to occur in the power supply between Line and the earthed chassis, the fuse would be out of circuit and not open.  Even the simple use of a mounting screw that is too long could cause this!

Figure 5b: Incorrect AC connection to a single fuse power supply

Always ensure that adequate inspection and testing techniques are in place with AC wiring.

Power Guy