04 Oct PQ Survey – Part 4
AC Electrical Sub-Panelboards Investigations
Earlier this year, iGround presented a series of blogs to address the methods of approaching the various levels of Power Quality site surveys, depending on the location to be investigated.
This latest installment focuses on the AC Distribution Panelboards, specifically sub-panels.
In Part 3 of this series, we addressed the issues related to investigating the AC Grounding Electrode (Earthing) System. In that installment, one of the things we DIDN’T address was the environmental aspect of being outside while digging up/testing the electrode system; namely the weather (extreme hot/cold), fire ants, spiders, snakes, alligators, tarantulas, more snakes, rats, and other nightmarishly dangerous things (did I happen to mention snakes?).
The danger INSIDE of a building is even less forgiving…electric shock. So, I want to make sure the reader understands that any metering investigation of a panelboard, regardless of its location, requires the participatory efforts of a trained professional (namely, an electrician) with the appropriate Personal Protective Equipment (PPE). Make sure you have updated and approved PPE if you are going to assist in any manner, including supervisory.
(Review OSHA 3151-12R or National Fire Protection Association’s NFPA 70E, Standard for Electrical Safety in the Workplace. Furthermore, your locale may have other requirements in place.)
Level 1 Survey
A Level 1 survey for an AC sub-panel will be broken into three subcategories; Visual, Mechanical, and Electrical (metering).
Recommended Tools and Equipment to have:
- Aforementioned PPE
- Appropriate screwdrivers and wrenches
- Torque wrench to check tightness
- True RMS AC Volt-Ohm-Milliammeter (VOM)
- True RMS AC Clamp-on Ammeter
Level 1 – (Visual)
Panelboards are available in many shapes and sizes so how we review the internal wiring may change our perspective, as well. Here are some of the things that I look for:
- Improper neutral-ground bonding. Those familiar with electrical codes are well aware that, for safety reasons, neutral and ground can only be bonded at two locations: the main service entrance or the source of a separately derived system (e.g., isolation transformer). Finding improper N/G bonds will require the use of a good set of eyes and a flashlight. Sometimes an improper N/G can take the form of a metal strap between the neutral bus and panelboard frame OR a discernably different GREEN screw that is connect to the panelboard frame behind the neutral bus.
(See Figure 1 for example of improper N/G bond in panelboard.)
I would also make sure to point out any white wires on the ground bus or green wires on the neutral bus. Having either of these conditions creates a N/G ‘reversal’ for the affected branch circuits and, thus, can be just as dangerous.
- If an IG busbar is mounted in the panelboard, be sure you verify that the equipment grounding conductor that references that busbar originates in the same conduit as the feeder conductors that enter the panelboard. If it doesn’t, you have a SERIOUS safety issues. Please refer to our previous blog, Facts and Myths of Isolated Grounding – Part 1, for more information.
Visual cues that may seem like minor housekeeping things: discolored terminations or conduit bushings (evidence of overheating), proper conductor identification (particularly of the feeder conductors), or more than one hot conductor on a breaker are worth noting.
Level 1 (Mechanical)
I cannot overstate the importance of tightening up connections at the main, the circuit breakers, and/or the busbars. It’s tedious, but your equipment will thank you for it.
Level 1 – (Electrical Metering)
The attached checklist should be self-explanatory in regards to ‘what’ we want to test for…but it’s going to hinge on experience and intuition, and some further knowledge of the power system, as to how we determine IF there are problems. Some rules to consider:
- An imbalance of current measurements between the feeder phase conductors is not a cause for concern if the upstream power source (transformer) is not loaded more than 40% of nameplate rating. Sure, there is a heat differential, but it’s likely NOT to be significant at ‘minimal loading’.
- Be sure to also measure current on the individual circuit breaker positions.
- Make current measurements on the feeder neutral, at a minimum. If the feeder neutral current is higher than the average feeder phase current, you can expect harmonic current to be present, requiring further investigation. To find the EXTENT of harmonic current, you can measure ac current with an average-responding meter and a True RMS meter. If the True RMS measurement is higher than the average responding meter, you can divide the two numbers to find out the harmonic current distortion.
- As a rule, I measure for current flow on any (and every) equipment grounding conductor within the panel…not just the feeder. This practice has led to finding other problems that could have been otherwise overlooked.
- Finally, if there’s an ‘isolated grounding’ (IG) busbar in the panelboard, verify that the equipment grounding conductor feeder is run with the rest of the electrical feeders (hot/neutral) (Figure 2) and NOT outside the panel to another ground rod.
<<Figure 2 with caption ‘Isolated Grounding (IG) busbar is referenced by a conductor run outside the panelboard frame. This is a safety and lightning damage concern…and often overlooked.’
Level 2 Approach
If I’m working alongside the customer’s electrician, I am always going to have a power line monitor that I can connect to an electrical panelboard that supplies power to sensitive equipment areas. Each power line monitor is different so the best advice I can give you is:
- Become familiar with the instruction manual and sampling rates for any device you use. Not all devices are created equal.
- Know the tolerances of the equipment before you begin setting monitoring ‘thresholds’.
- Fully utilize a power line monitor’s inputs. If the meter has five voltage inputs and four current inputs, utilize ALL of them. (See figure xxx for an example).
- The best-case scenario is to monitor the power at that panel for 7 full days, including weekends. Only then can you get a good idea of usage and repetitive disturbances.
- Have the customer keep an error log to correlate equipment/operational issues with your captured disturbances. Make sure you coordinate the customer’s clock with your PQ monitor’s.
- You may have to use more than one PQ monitor. For example, one each at the equipment location AND the subpanel will help determine if the PQ problem is at system-wide or localized.
Level 3 Surveys
The only marginally acceptable Level 3 testing that I can see doing at a panelboard is EMI measurements to make sure there is less than 1mG (milligauss) per meter…….and that is ONLY IF the panelboard in question is within six (6) feet of any sensitive electronic equipment device. Otherwise, Level 3 could be a waste of time here.
Infrared thermography, though popular, is a superficial (albeit safe) method of trying to find looses connections. However, since an IR meter will not detect loose ground connections (due to lack of current across the terminations), you have to open the panelboard, anyway. My suggestion is…dump the IR meter and get an electrician with a good set of wrenches, screwdrivers and appropriate PPE. Work with them to make useful measurements and you’ll get to the problem quicker.
As with other areas of this blog, retrieve a sample checklist, above, in the section for Recommended Tools and Equipment. It will cover all the things you may expect to encounter at a panelboard (some we’ve discussed here), but feel free to personalize it for your own needs.
PQ Surveys – Part 5
AC Outlets and Equipment Locations