Why is 2.5 milliohms (or so) specified for electrical bonding resistance? And how can such low values be measured?

23 Jun 2025

By Keith Armstrong, www.cherryclough.com, 30 April 2025

An electrical bonding limit value is a requirement of most military and aerospace standards, because their vehicles are exposed to harsh mechanical, chemical and climatic conditions that can loosen or corrode bonds that are important for safety and/or for EMC.

This means that confidence checks, refurbishments, repairs and replacements need to be able to be carried out in the field, where repeatable EMC testing is very difficult to perform.

So: we need a quick, easy, and low-cost way of checking the integrity of electrical bonds is being maintained within acceptable limits during an operational lifetime. And also, to ensure whether any work that has been done on any electrical bonds is likely to have compromised EMC (e.g. by using incorrect parts / materials, incorrect assembly procedures, or inadequate workmanship).

It has been known for decades that there are many problems and uncertainties in relying solely on the DC resistance of electrical bonds when trying to ensure EMC, especially at frequencies above 100kHz or so where the series inductance, rather than the series resistance, usually dominates the EMC performance of an electrical bond.

With very high bond currents flowing AC or RF, this series inductance can cause arcing/sparking across a bond, which usually causes significant EMC problems.  (Same problem occurs with too-high bond resistance when DC currents are high.)
Also, at very high frequencies, it is possible for an electrical bond to resonate, creating bond impedances that are essentially infinite (at that frequency) regardless of the size and weight of the metal parts used to construct the bond. If you prefer, you can characterise a resonating electrical bond as a type of narrowband RF antenna.

Over the decades, various inductance measuring instruments and other EMC measuring instruments and techniques have been proposed for field use to help solve this 'field checking and repair' problem, but – as  far as I know – they are not yet widely used in practice, and none are yet mandated by any EMC test standards (perhaps because of cost). I have several references on these I can send you, if you want.

It is important to understand that any specified maximum values of bond resistance (typically in the range from 2.5 to 10 milliohms) only apply when they are measured according to the relevant standard.

Also, please beware of your (or your customer) misunderstanding the ground bonding requirements in the EMC test standards. Often (e.g. in MIL-STD-461G and RTCA-DO-160G) these requirements only apply to the bonding resistances of the EMC test gear during EMC testing, to help ensure test repeatability. They probably do not apply to the ground bonding of the equipment concerned – which should almost always be installed in the test chamber as it would be installed in real life applications!

For example, according to Interference Technology’s “2022 MILITARY & AEROSPACE EMC GUIDE” (from www.interferencetechnology.com) both MIL-STD-461G and RTCA-DO-160G standards specify that only the installation provisions that are included in an equipment’s design or installation instructions should be used to connect the EUT to the chassis in a test chamber.

The way to avoid this is to find out where your (or your customer’s) specifications come from, then reading the appropriate documents carefully. Usually, our customers are pleased if we can point out that they have misunderstood a specification in a way that creates unnecessary costs.

I'm sure you already know that measuring such low resistance values:

              i)            Needs special milli-ohm-meters rather than ordinary multimeters,

              ii)           Needs special test leads and probes,

              ii)           Requires well-defined practical procedures that give repeatable results.

If someone is specifying a particular value (e.g. 10 milliohms) to you, it should have been derived from a particular test standard such as the USA’s MIL STD 461 or the UK’s DEF STAN 59-411 Part 5 (and before that, from DEF STAN 59-41 Part 7).
So: you should always make sure to ask them which clause in which standard they want you to apply when making this measurement against their specification.

This should ensure that you make the measurement correctly, helping to avoid upsets later on when someone else measures your equipment’s bonding resistances and your customer refuses to pay for your equipment (or insists on a costly redesign) because they got unacceptably high values merely because they did these tests differently from how you did them!

If the customer/specifier doesn’t know which clause in what test standard to apply, I very strongly recommend that you agree the test standard and relevant clauses to be used to measure bonding resistance with them, and get the resulting specification agreed at the very highest managerial level between you and your customer before including it in the official contract documentation.

It is important to check that all such technical specifications are fully, completely defined before agreeing a contract with a customer. The DC electrical resistance of a bond or connection sounds like an easy thing to measure, but it is not if it is below 100 milliohms.

I know several projects, each worth many millions of US Dollars / Euros /GB Pounds, that were seriously compromised, even leading to bankruptcies, because of such apparently simple electrical assembly issues. 

And yes, the electrical resistance of a bond requires that no extraneous current is flowing in it. This is usually easy to do on a stand-alone item of equipment by removing all of its power sources (AC and DC) and waiting for its capacitors to fully discharge.

But it gets progressively more difficult to do as systems and installations get larger, not least in the field because of stray currents induced in the metal structures by their local electromagnetic environments.

There are ways of overcoming such difficulties, including:

a) Auto-reversing measurement equipment that aims to cancel out measurement errors caused by extraneous quasi-static / DC currents and/or by thermoelectric effects (e.g. the “Seebeck” effect, where junctions between dissimilar metals converts temperature differences into electricity).      

b) RF measuring instruments that average out the errors caused by extraneous radio-frequency currents.  
(Most ‘ordinary’ analogue and digital meters/multimeters will demodulate RF noises, causing ‘zero-offset’ errors in their readings.)

c) Using measurement procedures and techniques that try to identify whether extraneous (noise) currents/voltages are present, for whatever reasons, and if they could cause unacceptable levels of measurement errors.

Here are some suppliers of ‘milliohm’ bond resistance test instruments, from Chris Nicolas, cnicholtwo@gmail.com:

Megger                                  DLRO 10

Chauvin Arnoux          CA6240

AGI                                            1681B
 An ATEX-certified bond-resistance meter for aircraft, which uses a very low AC current  at around 10Hz.

TESTFUCHS                     MVP10L-FS, MVP10R-FS
DC up to 10A, 2 or 4-wire test, allows impulse current testing, and automatic polarity reversal

PA-MVP11
DC up to 2,000A

IM2-FS
Loop Resistance Tester, up to 1A at 1kHz

BLRT2, ESNBLRT2KIT
Bonding And Loop Resistance Tester for aircraft,
very wide range of test capabilities

BLRT3
Portable Bonding And Loop Resistance Tester for aircraft,
up to10A DC, up to 1A at 1kHz

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