Touch Potential Safety Thresholds (B)

If you are responsible for protecting yourself or others against the effects of electric shock then you will probably be familiar with the term ‘Touch Potential Safety Threshold’.

But do you know what your Touch Safety Potential Threshold should be? Do you know who sets it?  Do you know how it was calculated? And importantly did you know the Threshold you use can affect not only your Safety, but also your Productivity?

Touch Potential Safety Thresholds

 

Brent Hill Volt Stick CIE

Before we start, I need to explain that I work for Volt Stick, and as a company we manufacture a Non Contact Voltage Detector that is widely used in the Utility Industry. The Volt Stick LV50 is an Intrinsically Safe Detector that detects from 50Vac and is used as a safety device to detect potentially dangerous Stray Voltages on metal Pipework and surfaces.

My reason for researching and writing this article came from talking to dozens of Safety Managers and discovering that what I assumed to be a universal safety standard was actually not the same everywhere and was sometimes different in parts of the same country.


If you are responsible for protecting yourself or others against the effects of electric shock then you will probably be familiar with the term ‘Touch Potential Safety Threshold’.
But do you know what your Touch Safety Potential Threshold should be? Do you know who sets it?  Do you know how it was calculated? And importantly, did you know the Threshold you use can affect not only your Safety, but also your Productivity?

What is a Touch Potential Safety Threshold?

If you’re not familiar with the term, then a ‘Touch Potential Safety Threshold’ is a Voltage Level that is regarded as ‘Safe’. Usually, official health and safety guidelines will be based around this threshold to help design safe working practices which will protect people from any hazards arising from the effects of electricity in the work place.

Often these guidelines will be enforced by regulations and, if you are following these guidelines and regulations, you would usually be doing enough to safeguard your workforce and to comply with the local law.

What is the ‘correct’ Touch Potential Safety Threshold?

This is where it gets interesting; whilst speaking to various safety managers at a trade show in Washington DC, it became clear that different safety thresholds were being used by different companies, yet these companies were doing very similar work; so why would their engineers be using different safety thresholds? 

At the trade show, I was aware of 3 Touch Potential Safety Thresholds being used. I was already familiar with the threshold set by the UK’s Health and Safety Executive (HSE); the others were the National Association of Corrosion Engineers (NACE) and the Occupational Safety and Health Association (OSHA) which are both used in the US.

The HSE and OSHA had set 50V Safety Thresholds, whilst NACE had set theirs at 15V. Even though the engineers using these safety thresholds were all doing very similar jobs, they were all convinced that the safety threshold they were using was the ‘right one’.

To find out who was right I decided to research each of the HSE, NACE and OSHA's Thresholds.
I discovered each of the Thresholds had been derived using data from independent experiments and reports that had yielded very similar results. The general principals used to calculate each of these thresholds were the same; so why would this result in different Touch Potential Safety Thresholds being used?  To answer this question we must first understand how a Touch Potential Safety Threshold is calculated...


How is a Touch Potential Safety Threshold calculated?

Safety Thresholds are usually based on the measurement of the Current, which results in a range of physiological responses - and that data is then translated into a Voltage Safety Threshold. The calculation can be quite complex and can include the path of current, the duration of contact, the conditions (as in wet or dry) and many other factors. The calculation can be modified to suit many different situations and conditions and can be used to calculate thresholds to protect against various different physiological responses.

It’s important to note that one threshold shouldn’t be regarded as better than another, nor should one simply adopt the threshold that others use without first understanding the conditions and physiological response that that threshold is protecting against.

Touch Potential Safety Thresholds should be calculated taking into consideration each individual scenario and the expected environmental conditions.

To better understand how a Safety Threshold is calculated, lets break down the calculation into its various components and see just how a Current-based Threshold is converted in to a Voltage-based Threshold.

What are the components that make up the calculation?

The Calculation, simplified is basically V=IR (Ohms Law),  Voltage Threshold = Current Threshold x Body Resistance

Voltage Threshold Equation

So if we want to know the Voltage Threshold, then the starting point of the calculation is to find the Current-based Threshold.
To get a Current Based Threshold, the HSE, OSHA and NACE referred to scientific reports generated by scientific bodies and universities. These bodies have documented the effects of Current on human beings and identified the various physiological responses and the level of Current that it takes to induce those responses.

There are 4 recognised physiological responses from electrical current, which are:

  1. Threshold of perception (Tingle Sensation)

  2. Startle Reaction  (Small Shock)

  3. Muscular Reaction (Inability to let go)

  4. Ventricular Fibrillation (Heart Failure)

To begin the voltage safety threshold calculation, a decision must be made as to which physiological response the voltage threshold must protect against. Do you need to protect against Ventricular Fibrillation or perhaps your engineers mostly work at height so you may need a lower Threshold to protect against a Strong Muscular Reaction which could make someone fall from a ladder.

But then as stated in some guidance, secondary actions like falling etc can be safeguarded with other physical procedures like harnesses and safety ropes etc. Interestingly, it is from this point that one of the safety bodies interprets this data differently to the others and arrives at a different safety threshold, but we will come back to this. (*A)

Once the Current Threshold has been decided, we can move on to calculate the Voltage Threshold using V=IR. So if V is the Voltage Threshold and I is the Current Threshold, then R is the Resistance of the Body.
The same scientific Bodies that studied physiological responses also calculated the resistance of the body and this is where the calculation gets a little more complicated. When calculating the Resistance or Impedance of the Body there are more considerations to be made, the Impedance was found to be affected by the following –

  1. The Path of Current through the body

  2. The Skin Contact Area

  3. The Wetness of the Skin

  4. The Voltage Applied

  5. The Duration of the Current Applied

We can see that Body Impedance can be affected by many factors, so when calculating the Voltage Threshold a decision needs to be made as to what the most likely conditions would be, taking into consideration the likely skin contact area and moisture and the likely path of current through the body.


So why are the HSE, OSHA and NACE Touch Potential Safety Thresholds different?

Now we have a better understanding of the calculation, we can try to find out why there is a difference between the Touch Potential Safety Voltages as recommended by the HSE (UK), NACE (US) and OSHA (US).

In the next section we’ll take a closer look at how each Threshold has been calculated.

  • UK- HSE (Heath and Safety Executive)

The UK Government Safety Guidelines and Regulations are set by The Heath and Safety Executive (HSE).
Guidance HSG85 provided by the HSE, states that a ‘dangerous’ Voltage Potential is 50Vac or 120Vdc in dry conditions and Regulation HSR25 makes requirements to’prevent danger’ or ‘prevent injury’. Both the HSG85 and HSR25 documents refer to reports made by the IEC.

  • IEC (International Electrotechnical Commission)

The IEC is an international standards organisation that prepares and publishes International Standards for all electronic and related technologies (Electrotechnical). The IEC was founded in 1906 with representatives from Austria, Belgium, Canada, Denmark, France, Germany, Great Britain, Holland, Hungary, Japan,  Norway, Spain, Sweden, Switzerland, and the United States. The aim of the IEC is to develop and distribute standards and units of measurement so as to unify terminology relating to electrical, electronic and related technologies. Today the organisation is based in Geneva with over 160 Member Countries that use their standards.

IEC 60479-5 – Touch Voltage Threshold Values for Physiological Effects – a report referred to by the HSE.
This technical report provides touch voltage- duration combination thresholds based on the analysis of information concerning body impedances and current thresholds of physiological effects that were reported in IEC/TS 60479-1.

The reports address the 3 key physiological reactions:

a) Startle Reaction - minimum derived value of touch voltage for a population for which a current flowing through the body is just enough to cause involuntary muscular contraction to the person through which it is flowing.

b) Strong involuntary muscular reaction – minimum derived value of touch voltage for a population for which a current flowing through the body is just enough to cause involuntary contraction of a muscle, such as inability to let go from an electrode (a.c.), but not including startle reaction.

c) Ventricular Fibrillation – minimum derived value of touch voltage for a population for which a current flowing through the body is just enough to cause ventricular fibrillation.

When calculating the Body Impedance the report takes into consideration the path of Current and the size of the contact area and whether the contact area is wet or dry. From the results, the flow chart below was produced to direct people to the right chart so that they can calculate their own Touch Potential Safety Threshold.

Voltage Threshold flowchart

If we remember from above, the HSE states 50Vac in Dry Conditions as their Touch Potential Safety Threshold, so if we look at the flow chart above then this would direct us to Fig 11, Fig12 or Fig 13 depending on the size of the contact area that the HSE were anticipating.

The IEC Report uses the following contact areas:

  1. Large Contact Area – 82 cm² = large contact area
  2. Medium Contact Area – 12 cm² = might represent touching a conductive part in the palm of each hand
  3. Small Contact Area – 1 cm² = might represent touching a small conductive part with the hand

The following charts are Fig11, Fig12 & Fig13 as referred to in the above flow chart. Each chart plots Voltage against Time and the resulting Physiological Responses...

Figure 11 Conventional time/voltage zones of effects of a.c. current (50/60 Hz) on a person for dry condition and large contact area

We can see that a Large contact area of 82cm² requires a Safety Threshold of 30v to protect against Ventricular Fibrillation, even at long durations we can see that 30v would not cross the Ventricular Fibrillation line.

 

Figure 12 Conventional time/voltage zones of effects of a.c. current (50/60 Hz) on a person for dry condition and medium contact area

A Medium Contact Area of 12cm² requires a Safety Threshold of 60v to protect against ventricular fibrillation; it can also be seen from this chart that even with a contact time of 10 seconds 60Vac would not cause Ventricular Fibrillation.

 

Figure 13 Conventional time/voltage zones of effects of a.c. current (50/60 Hz) on a person for dry condition and small contact area

A Small Contact Area of 1cm² requires a Safety Threshold of 90v to protect against ventricular fibrillation

 

We can now see by using a 50v Touch Potential Safety Threshold that the HSE Guidelines would protect from the danger of Ventricular Fibrillation for contact between a medium and large area in Dry Conditions.

IEC/TR 60479-5 – suggests that not only the above be taken into consideration when determining safety limits, but also a risk assessment should be made that includes the reduction of the likelihood of contact by the use of obstacles, barriers and warnings, the reduction of touch voltage by using bonding etc., and additional resistance provided by protective clothing such as gloves and shoes.

  • US - OSHA (Occupational Safety and Health Association)

The OSHA is part of the United States department of Labor and sets Electrical Safety Guidelines that are used by most of the US.

OSHA 1910.269 is a Guideline that sets out procedures for safe working around Electrical Hazards and advises the use of MADs (Minimum Approach Distances). In Table R-3 and R-6 for AC Voltages, the Thresholds start at 50Vac and the advice is to ‘Avoid Contact’with voltages above 50Vac.

OSHA 1910.269 refers to the IEEE (Institute of Electrical and Electronic Engineers) Standard 1048-2003 which is the Guide for Protective Grounding of Power Lines. In a very similar way to the IEC reports used by the UK’s HSE, the IEEE determine thresholds that will protect against Ventricular Fibrillation using complex calculations that also take into consideration the Body Impedance and duration of contact.

The Current and related physiological response data for the IEEE calculations were derived by experiments carried out by Charles F. Dalziel at the University of California in Berkley, California, between the 1940s and 50s and his results are summarised in the following table.

quantitative effects of electric current on man

By knowing the Current Flow that would result in certain physiological reactions for 99.5% of the population, a value of resistance for the ‘average’ human could be calculated and a formula devised  to give a Voltage Threshold  to protect against Ventricular Fibrillation.
The formula is known as Dalziel's Formula and would have been used by the IEEE to help calculate and set the OSHAs Touch Potential Safety Threshold to 50Vac.

It is also worth noting that another recognised US Safety Standard, the National Electrical Safety Code (NESC) is also published by the IEEE and uses the same Minimum Approach Distances starting at 50Vac.

  • US - NACE (National Association of Corrosion Engineers) (15v Threshold)

NACE is a Worldwide Corrosion Authority that was established in 1943 by corrosion engineers from the Pipeline Industry. The organisation provides standards designed to protect people, assets and the environment from the effects of corrosion.
NACE RP0177-2000 is a Standard for the Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems, and sets its Touch Potential Safety Threshold at 15Vac.

Section 5 – Personnel Protection
Quotes work done by George Brodier at Columbia University for showing that a reasonable value for Body Impedance for the purpose of estimating body currents is 1500 Ohms hand to hand or hand to foot.
The Standard also quotes Dalziel’s Current Data for Physiological Responses and uses a value of 10 Milliamperes as a maximum safe let go current.

Using  V=IR (Voltage = Current x Resistance)
Voltage = 0.010 x 1500 = 15Volts
Nace is able to set it’s Touch Potential Safety Threshold at 15V

Touch Potential Safety Threshold


Conclusion

To reiterate, the above are my own thoughts, questions and conclusions, I do not claim to be an expert but I would like to get the people that are experts to discuss the points that I’ve raised and to correct me if I am wrong!

We can see that both the HSE and OSHA have used independent research and calculation methods and both have reached a Touch Potential Safety Threshold of 50Vac to protect against Ventricular Fibrillation even with long contact times.
(*A) The data and methods used by NACE were very similar to the HSE and OSHA but the fundamental difference was that NACE has chosen to base their Touch Potential Safety Threshold around the ‘Safe to Let Go’ Current Threshold rather than Ventricular Fibrillation and hence the lower Touch Potential Safety Threshold of 15Vac. The choice to do this could have something to do with the type of work or scenarios that Pipeline workers may find themselves in and so might need to be protected against a ‘Strong Muscular Reaction’ rather that Ventricular Fibrillation?

From looking at the data and the calculations, my conclusion is that the 50v Threshold protects against Ventricular Fibrillation even at longer contact times. In fact a senior member of the IEEE, D.Dorr states in his paper 'Determining Voltage Levels of Concern for Human and Animal Response to AC Current’ that “no known fibrillation deaths had been documented with voltages of 50Vac or less.”

So it seems to me to be over-cautious to use a threshold of 15Vac, it will protect against being unable to 'Let Go’, but as long as that Voltage is below 50V then there should be no danger of Death from Ventricular Fibrillation. Also, if we look back at IEC 60479-5 Fig 1, we can see that even with a Large Contact area in dry conditions that the Voltage would need to be in excess of 30Vac for there to be a danger of Ventricular Fibrillation. In fact, looking at the same chart someone would need to be in contact with a Large Area (82cm2) for almost 1 Second before being in any danger of not being able to let go.

If the 15v Threshold is there to protect against secondary injuries caused by a Muscular Reaction then this too could be protected against by putting in other physical safety procedures as recommended by IEC/TR 60479-5

The utility companies that I spoke with at the Trade Show were all doing very similar work yet some were using a 50v Safety Threshold and others a 15v Safety Threshold. As stated previously, the 50V Threshold isn’t better or worse than a 15V Threshold but it is crucial to understand how each were calculated and why they were set at a particular level. Using a lower Safety Threshold could inhibit the type of work that can regarded as safe and therefore could affect productivity.

Volt Stick LV50 non contact voltage tester

Bringing this back to where we started and the use of Non-contact Voltage Detectors to check for stray voltages. It is important that whatever touch Potential Safety Threshold that you are using, make sure that the Non-Contact Voltage Testers you are using are, in fact, detecting from that threshold voltage.

Also remember that the lower the Voltage Threshold will require more sensitivity from the Non-Contact Detector and the more sensitive a detector is, then the less accurate it becomes. If a Non-Contact Detector is so sensitive that it will detect 15Vac then it will also detect any voltages above 15Vac, so voltages of 110Vac or 220, 230 or 240Vac will be detected from a significant distance, so in some areas it would become very difficult to use a very sensitive Non-contact Voltage Detector. Therefore, not using the right safety threshold for the type of work you are doing could not only affect your Safety, but also your productivity!

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