So I have been researching creepage and clearance for a few weeks now and I think I have a pretty good understanding of the concepts. However what I do not understand is some of the interpretations of the standards.

My Question: When UL 61010 standard states a voltage, is that the difference between the two adjacent conduction pairs or is that voltage relative to ground. I think it would be relative to ground but thought I'd ask here.

I am asking because theoretically you can have 2 1000V lines very close to each other and from what I understand there is no potential difference to cause an arc, however I highly doubt that you would pass any certification doing this.

  • \$\begingroup\$ It's the maximum possible voltage differential between the two pairs under normal operation, I believe. If your two 1000V lines are shorted together somewhere you could put them right next to each other, but if they're separate systems, you should separate them assuming one of them may be turned off and at zero volts. (disclaimer: This is just from my knowledge of best practices, and I am not intimately familiar with the UL standards. Take what I say with several grains of salt.) \$\endgroup\$ – Hearth Nov 28 '18 at 17:14
  • \$\begingroup\$ Are the lines in the same phase? If there is a phase difference, there is also potential difference. \$\endgroup\$ – TemeV Nov 28 '18 at 19:31
  • \$\begingroup\$ describe what you mean by "very close": and "certification" for what? show a picture link or the application. Otherwise it is vague. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 28 '18 at 21:13

My understanding is that clearance and creepage requirements in safety standards apply for the separation between user-accessible parts and potentially hazardous circuits, not between two hazardous circuits.

I have access to the IEC 61010 standard, which I believe is very nearly identical to UL 61010. In IEC 61010-1, section 6.4.2, the requirement is

If ENCLOSURES or PROTECTIVE BARRIERS provide protection by limiting access, CLEARANCES and CREEPAGE DISTANCES between ACCESSIBLE parts and HAZARDOUS LIVE parts shall meet the requirements of 6.7 and the applicable requirements for BASIC INSULATION. [emphasis added]

and in section 6.4.3

CLEARANCES, CREEPAGE DISTANCES and solid insulation forming BASIC INSULATION between ACCESSIBLE parts and HAZARDOUS LIVE parts shall meet the requirements of 6.7. [emphasis added]

Since presumably neither of your two 1000 V wires is accessible, these requirements don't apply, and not meeting the clearance and creepage distances between these wires shouldn't prevent you from meeting a 61010 standard.

However, you might still want to prevent arcs between these two circuits (for example, when one is powered but the other one isn't), and the clearance and creepage values from 61010 may be a reasonable guide to how to do that.

One point in IEC 61010 that I'm not entirely clear on the interpretation of is in section 6.7:

Insulation between circuits and ACCESSIBLE parts (see 6.2) or between separate circuits consists of a combination of CLEARANCES, CREEPAGE DISTANCES and solid insulation.

This would tend to indicate that creepage and clearance should be observed between "separate circuits", but I don't see any "shall" clause saying when this must be provided.


I am not advising you on whether your design is safe and you should have your design checked by a licensed engineer in your jurisdiction before you offer it for use by the public.


Creepage and Clearance are terms used for gaps in Electrical Insulation between electrodes or conductive end-points and not relative to Earth ground unless that happens to be one of the conductive surfaces. This is true for any document including UL 61010.

Creepage is more prone to a lower breakdown voltage so the surface ionic contamination of dust and moisture requires a much bigger gap which is sometimes rated according to location.

Clearance usually refers to air gaps between conductors or partial conductors, which may be rinsed away or vacuumed by preventive maintenance good practice.

Other Info

These two terms have resulted in a wide variety of insulator bushings designs in order to enhance the creepage path length and clearance gaps in certain standards. The most prevalent test method is a lightning surge test at some level and rise/fall time. This test acceptance level along with the pulse shape is used in the Power Industry to define a Basic Insulation Level or BIL, e.g. BIL200 means it can survive a 200kV lightning impulse. Yet it would fail miserably with 100kV AC at low frequency and in my experience fail even if cleaned with alcohol wipes at 65kV. Insulation can withstand higher peak voltage transients if they are faster than the ionization time needed to ignite air or the insulation defined by creepage and clearance.

Notice the Clearance between rings and the depth of the rings adds to Creapage path length are different in each case depending on materials and environment risks (sand, dust, condensation, rain frequency) so one size or shape does not suit everyone. enter image description hereenter image description here

Usually, when there is a weak arc, it cleans the surface of contamination. A strong arc might add a carbon trail and make it worse.

If you are keen on this topic there are tens of thousands of thesis results on "Partial Discharge" I picked one graduation project at random... you may find easily

Analysis of Partial Discharge in OIP Bushing Models ZEESHAN AHMED Stockholm, Sweden 2011 XR-EE-ETK 2011:008


There are two basic considerations regarding potential differences in standards.

  • Possible potential differences present during normal operation.
  • Possible potential differences present during reasonable fault conditions.

IEC60601, being a medical standard, has requirements for both.

If during normal operation both 1000V circuits, always have the same potential with respect to each other AND there is no normal condition that violates this then the normal operation requirements apply.

If during a single or double fault condition a large potential difference can exist between those two circuits, then the fault conditions apply (this can simply be a requirement to ensure that a fuse is blown).

Keep in mind that normal for IEC60601 implies the presence of line voltage on any exposed part and thus across any isolation barrier in the system AND the presence of up to 5kV for 1 minute across any isolation barrier without significant system degradation. Additionally, UL would require appropriate response to a static discharge model that has voltages of ~10kV.

Disclaimer: Always get the required certification for equipment that goes into the market and ensure best practices for equipment that comes into contact with humans. The certification process includes the verification of those isolation barriers.

  • 1
    \$\begingroup\$ Note that OP asked about 61010 which is a standard for equipment for "measurement, control, and laboratory use", not for medical equipment. \$\endgroup\$ – The Photon Nov 28 '18 at 21:53

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