Interpretation of EN 61000-4-5 Surge requirement on a DC Power Port

Question

I am trying to understand how to treat the DC power port on my industrial device (read: will be used in CNC machine shops and industrial processing plants and the like) should be tested according to EN61000-6-2 for the surge test specified in EN 61000-4-5.

Table 3 in the standard calls out the tests required for DC power ports. Line 3.2 calls out 500V surge testing applied between line to ground and also between line to earth. So far there is nothing confusing.

However, note (c) is called out under "remarks" for surge testing, and it states that surge is not applicable under certain conditions. Most of the conditions don't apply to my situation, but the final line of the note states "DC Ports, which are not intended to be connected to a DC Distribution network, are treated as signal ports".

So now the question is - what is a DC Distribution network? My device is intended to run off of either 12 or 24 V DC power, but it's unclear to me if a voltage regulator on the same CNC machine or process equipment as my device is considered a "network", or if it must be some centralized regulator that is run throughout a plant, or some other definition. Can anyone here provide a working definition or clarity on what this means?

If I can treat the DC Power port as a signal port, it would greatly simplify my transient protection scheme.

Answer 1

The surge test according to IEC EN 61000-4-5 is performed on equipment whose power supply connections are geometrically far away from the device itself, in order to test its resistance to the three following stroke related events (accordig to this nice survey):

1. Direct lightning strokes to outdoor circuits injecting high currents and producing overvoltages.
2. Indirect lightning strikes, i.e. strikes between or within clouds or to nearby objects which produce electromagnetic fields that induce voltages/currents on the conductors outside and/or inside a building.
3. Lightning ground current flows, resulting from nearby direct-to-earth discharges coupling into the common ground paths of the grounding system of the installation.

Note that all three events above involve long cable lengths and distances: specifically the third one, which is closely related to the necessity of making a 500V surge test, requires the presence of a long ground path in order to induce a dangerous EMF on nearby cables by high value pulsed currents flowing through it (by the Faraday-Neumann-Lentz law).

Said that, if you equipment is powered from a DC power supply placed within a few meters (precisely within \$3\$ meters of cable, but I can possibly remember a wrong value) from it, the three events above are considered as very low probability events to happen on the DC power input: the DC port is thus considered as a signal port. Therefore you can avoid the IEC EN 61000-4-5 test on the DC input of your equipment, but

1. you must do it on the input of your DC power supply (or choose a a power supply tested accordingly) and ship it to be used with your equipment or
2. state a maximum cable length for the DC supply power cables (customarily less than the maximum one allowed).
3. Edit: something that I remembered later is that if your device is designed to be embedded inside a larger equipment, you can require your customer to provide you a nice power supply rail, fully protected from surges. Such devices are sometimes called OEM (Original Equipment Manufacturing) devices.

If you choose to not proceed according to the above guidelines, it is implicitly assumed that your equipment can be powered by a DC rail, involving a single power supply placed several (cable) meters away from the DC input of your equipment and which possibly powers other plant equipment as well: then you have to be sure that it survives a surge event, so you have to do the test.