New IEEE 1584™ -2018 wording: “Possible but less likely”
The IEEE standard 1584™ – 2002 Guide for Performing Arc-Flash Hazard Calculations stated, “Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.”
The recently published IEEE standard 1584™ – 2018 has removed this statement and provided an alternative in section 4.3:
“Sustainable arcs are possible but less likely in three-phase systems operating at 240 V nominal or less with an available short-circuit current less than 2000 A.”
The problem is that the wording “possible but less likely” does not provide an exact way forward. Within the next few weeks, we aim to publish a detailed article on the topic but decided to blog some information we have already.
How does this affect you?
Aside from all the changes in IEEE 1584 – 2018, this topic has a substantial impact on day-to-day operations since most workers have their hands on these types of equipment (e.g., 240V and 208V panel boards, disconnects, heaters, instrumentation power). Your previous program and/or labels may have stated that no arc-rated protection was required. Are those still valid?
Differences in Equipment
It is important to limit these discussions to three-phase industrial type equipment since pole mount transformers and power utility equipment behave differently.
Three-phase Industrial Equipment
What should be done if your plant has several pieces of equipment less than or equal to 240V that are indicating values lower than the onset of a secondary burn (typically indicated as < 1.2cal/cm² at 18 inches)?
- A colleague and I ran rudimentary simulations considering dry-type transformer primary voltages ranging from 480V to 13.8kV with transformer impedances between 3.5% to 7.5%. Secondary voltages were either 208V or 240V. Transformers less than and equal to 30kVA produced short circuit currents bordering on and generally less than 2000A. In such instances, you can continue following your program for transformers rated 30kVA and below.
What about larger transformers?
- Transformers rated 45kVA and above always produced more than 2000A. Here it becomes a tad more complicated in that the arc flash study assumptions must be known. Did the engineer (using the IEEE 1584-2002) default all equipment fed from transformers rated less than 125kVA (≤ 240V) as less than 1.2 cal/cm2? What we are getting at, is that the available energy is dependent on (amongst several other factors!) short circuit current and fault clearing time. It is possible that the main breaker limits the arcing time to keep those systems less than 1.2cal/cm2. That clearing time / short circuit combination must be calculated. The “default” option does not perform the calculation.
- If the engineer “defaulted” to the “less than 125kVA” option (as some studies have), a recalculation on that part of the study is required.
- If the engineer did not use that default, then you need to consider two areas:
- You can continue working without mandating arc-rated protection IF the area downstream of the main breaker is less than 1.2cal/cm2.
- The operation of the main breaker or any work that exposes a person to the secondary terminals of the transformer up to and including the main transformer breaker may likely require a level of arc-rated protection.
OSHA 1910.269(l)(8) and National Electrical Safety Code® (NESC®) – 2017 Tables provide guidance for pole mount and utility equipment.
Further reading on that topic can be found on our blog: OSHA 1926 Subpart V Differs With NESC 2017 on Arc Flash Calculations
NFPA Hierarchy of Control
An overarching principle is that workers should follow the hierarchy of control in the NFPA 70E®-2018. We do not recommend defaulting to an NFPA 70E® (any revision) rule and cross-pollinating it with the IEEE 1584™ -2018 (e.g., old rules including less 10kA, less 125kVA, etc.). The reasoning is simply because the latest and previous editions of the NFPA 70E® articles were based on IEEE 1584™-2002 which has since been replaced.
It is likely that the NFPA 70E®- 2021 cycle may miss these changes since the first round of public input has already closed.
At this stage, as mentioned above, the final risk control option may be arc-rated protection if no OSHA or NFPA 70E® -2018 acceptable alternative is available. Even then, such PPE requirements would be limited to the transformer main breaker (or main lug connection) operation in the 45kVA – 125kVA range. Since these are not operated frequently, using qualified contractors or qualified employees would be a reasonable alternative.
More Info Forthcoming
As these are very early days working with the new standard, this write-up will likely be updated. We are already working on a more detailed article with feedback from other IEEE 1584™-2018 working group members.
Feel free to share any arc flash burn incidents that you may have experienced on industrial systems fed from transformers rated less than 125kVA with nominal three phase voltages less than 240V. We have not come across such incidents and are looking to expand on our 200+ collective investigation experience.