{"id":3541,"date":"2026-10-21T13:18:39","date_gmt":"2026-10-21T05:18:39","guid":{"rendered":"http:\/\/www.eungabiho.com\/blog\/?p=3541"},"modified":"2026-10-21T13:18:39","modified_gmt":"2026-10-21T05:18:39","slug":"what-coordination-requirements-are-there-between-an-outdoor-rmu-and-other-power-equipmen-4c96-fa85c8","status":"publish","type":"post","link":"http:\/\/www.eungabiho.com\/blog\/2026\/10\/21\/what-coordination-requirements-are-there-between-an-outdoor-rmu-and-other-power-equipmen-4c96-fa85c8\/","title":{"rendered":"What coordination requirements are there between an Outdoor RMU and other power equipment?"},"content":{"rendered":"

Hey everyone, Jamie here from the Outdoor RMU squad. If you\u2019ve ever picked a power distribution part that looks like it could survive a zombie apocalypse and actually works, you\u2019ve probably run into our Outdoor RMUs\u2014they\u2019re not just metal boxes on poles that keep the lights on, they\u2019re the middlemen of the grid that make or break how smoothly a whole network runs. Last week, I got a random call from a small utility co-op in Ohio that basically said, \u201cWe popped in a new Outdoor RMU and now the whole feeder is tripping like crazy.\u201d Turns out they skipped a few tiny coordination steps between that RMU and their other gear, and they were stuck on the side of the road for 12 hours during a heatwave. That\u2019s why I\u2019m breaking down the exact coordination requirements you can\u2019t sleep on if you\u2019re working with Outdoor RMUs (especially if you\u2019re teaming \u2019em with other power equipment) straight from the supplier\u2019s mouth\u2014no vague whiteboard jargon, just the real stuff we fix every day. Outdoor RMU<\/a><\/p>\n

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First off, let\u2019s cut the confusion: an Outdoor RMU (Ring Main Unit) isn\u2019t your grandma\u2019s old switchgear. It\u2019s a sealed, weather-proof box that handles switching, protection, and metering on medium-voltage (MV) outdoor grids\u2014think areas with poles, suburban neighborhoods, rural lines, anywhere you can\u2019t fit a big indoor substation. The whole point of pairing one with other equipment is to stop outages, isolate faults fast, and keep power flowing to the houses (and warehouses, hospitals, farms) that need it. But when one gear talks over another, or their settings don\u2019t line up, that\u2019s when you get those silly tripping issues or even worse, equipment damage.<\/p>\n

Let\u2019s start with the big one that almost every new customer misses: coordination with downstream and upstream overcurrent protection devices. This isn\u2019t just \u201cset numbers and call it a day\u201d \u2014 it\u2019s a dance, and if your RMU steps on the toes of the breaker or fuse above it, you get what we call \u201cnuisance tripping,\u201d which is exactly what happened to that Ohio crew. Let\u2019s break it down: upstream protection is usually the main breaker at your substation or a recloser further up the line. Downstream is stuff like fuses, sectionalizers, and even customer-side transformers. Your Outdoor RMU has its own overcurrent relays\u2014we program these to trip when there\u2019s a short circuit or overload, right? But if that trip threshold is lower than the upstream breaker\u2019s, the RMU will trip first for a fault that actually should be handled by the upstream gear. That\u2019s like having a cop arrest someone for jaywalking when there\u2019s a bank robbery down the street\u2014wastes time and leaves the whole line dead.<\/p>\n

For example: Let\u2019s say you have an Outdoor RMU at the end of a 5-mile feeder, and the substation main breaker is set to trip at 10,000 amps. If your RMU\u2019s overcurrent relay is set to trip at 8,000 amps, any fault in the middle of the line will make the RMU blow before the main breaker even gets the signal, leaving half the feeder dead. That\u2019s bad. So what\u2019s the fix? We (as the supplier) always give a recommended coordination curve, but you\u2019ve got to match the RMU\u2019s time-current characteristic (TCC) with the upstream and downstream gear. Most of our new RMUs come with adjustable TCCs now, so you can tweak the trip time\u2014like, set the RMU to trip 0.2 seconds after the upstream breaker. That way, if it\u2019s a minor fault, the upstream takes over, and if it\u2019s a big one, the RMU steps in. We also work with a lot of utilities that pair their Outdoor RMUs with reclosers\u2014super common in windy, rural areas. If the recloser is set to auto-reclose 3 times after a fault, your RMU\u2019s protection has to wait until those recloser tries are done before locking out. Otherwise, the RMU will trip during the first recloser attempt, and you\u2019ll have a continuous cycle of tripping and re-closing that burns out equipment.<\/p>\n

Next up, voltage coordination\u2014this is the silent killer that causes a lot of hidden issues, not just big blowouts. Outdoor RMUs handle medium voltage, usually between 12kV and 36kV, and they\u2019re wired to match the system\u2019s nominal voltage, right? But if you pair it with other equipment that has different voltage ratings, or there\u2019s a voltage fluctuation (like a sudden load spike from a new mall being built down the line), your RMU can get fried, or the whole system goes wacky. Wait, I should clarify: voltage coordination isn\u2019t just matching numbers\u2014it\u2019s accounting for transient voltage surges too. That\u2019s where surge arresters come in, and you have to sync those with your Outdoor RMU. Let\u2019s say you put a surge arreder on the pole right next to the RMU, but it\u2019s rated for 24kV, and your system jumps to 28kV during a startup. The arreder can\u2019t handle it, so it fails, and your RMU\u2019s insulation gets punctured. We\u2019ve had customers try to cut costs by using cheap surge arresters that don\u2019t sync with their RMU\u2019s voltage class, and we get 2AM calls about smoke coming out of the RMU. Here\u2019s the rule we hammer home: always check the RMU\u2019s rated voltage (we label this clearly on the box, but you\u2019d be surprised how many people skip the spec sheet) and match every other piece of equipment on that node\u2014transformers, reclosers, arresters\u2014to that exact rating, plus a 10% buffer for transients. Also, if you\u2019re using a voltage regulator downstream of the RMU, make sure the RMU can handle the regulator\u2019s voltage adjustments. If the regulator bumps the voltage up by 5% to feed a neighborhood with heavy load, and your RMU\u2019s max is only 3% over nominal, that\u2019s a recipe for insulation damage over time. We had a solar farm customer a while back that didn\u2019t do this, and after 6 months, their Outdoor RMU\u2019s internal contacts corroded because of constant overvoltage\u2014they would\u2019ve saved $1k on a spec check instead of paying $50k to replace the RMU and fix the downstream transformers.<\/p>\n

Then there\u2019s communication and synchronization, which is huge for smart grids now. A lot of our clients don\u2019t realize that an Outdoor RMU isn\u2019t just a mechanical switch\u2014it has built-in communication modules for SCADA (supervisory control and data acquisition), smart meters, and even other grid devices. If that comms module doesn\u2019t talk the same protocol as the adjacent equipment, you can\u2019t remotely monitor faults, or the system won\u2019t sync during load transfers. For example: A suburban utility we work with tried to pair our new Digital Outdoor RMU with their existing sectionalizers, which used an old DNP3 protocol. Our RMU uses IEC 61850, which is the new standard for smart grids. They couldn\u2019t get the two to exchange data, so when a fault hit, the sectionalizer wouldn\u2019t isolate the bad line, and the RMU didn\u2019t send an alert to the control room. That meant a whole neighborhood was without power for 3 hours because no one knew where the fault was. The fix here isn\u2019t replacing gear\u2014we offer comms converters that sync old and new protocols, and we always include a comms compatibility check with every RMU quote. Also, load synchronization is a big one if you\u2019re pairing Outdoor RMUs with distributed energy resources (DERs) like solar or wind inverters. If you\u2019re switching the RMU to connect a solar farm to the grid, the voltage, frequency, and phase have to match exactly with the grid side, or you\u2019ll cause a blackout or damage the solar inverters. We have a step-by-step sync guide for all our RMUs, but we always do a quick test run with the DER supplier to make sure everything lines up. Last year, a small solar installer messed up the sync settings on our RMU and their inverters, and they had to replace 4 inverters\u2014costing them way more than the sync test would\u2019ve.<\/p>\n

Oh, and don\u2019t forget physical and structural coordination, which is way more important than it sounds for Outdoor RMUs. These things are mounted on poles, pads, or even in underground vaults sometimes, right? If you\u2019re mounting an Outdoor RMU on a pole next to a recloser, their clearances have to match so you can do maintenance without accidentally crossing lines. Also, weight and load rating\u2014if you put a 1,000-pound RMU on a pole that\u2019s only rated for 800 pounds, the pole will bend, and the whole setup will fall during a storm. We design our RMUs with different mounting options (pole, pad, vault) and list the exact weight and clearance specs on every product page, but some utilities just reuse old mounting hardware that\u2019s not compatible. We had a rural customer in Texas do that during a tornado last year\u2014their RMU wasn\u2019t secured right, so it slid off the pole and took out 3 miles of line. That\u2019s a mess no one wants to clean up. Also, insulation coordination for the outdoor environment: if your area gets a lot of salt (coastal) or pollution (industrial areas), your RMU\u2019s bushings have to be rated for that, and adjacent equipment\u2019s bushings too. Salt buildup causes flashovers, so if your RMU has salt-rated bushings but the downstream transformer doesn\u2019t, you\u2019ll get a flash that jumps between the two, causing a fault. We make salt-pollution rated RMUs, but we always remind customers to check every piece of equipment on that feeder for the same environmental rating.<\/p>\n

Wait, let\u2019s also address the \u201cinvisible\u201d coordination that happens between Outdoor RMUs and protective relays in other parts of the grid. I mentioned overcurrent earlier, but directional relays are a big one if your grid has bidirectional power flow (like with DERs). If your Outdoor RMU has a directional relay set to only detect power flowing downstream (from the grid to customers), and your solar farm is feeding power back upstream, the relay won\u2019t see the fault on the customer side, so it won\u2019t trip to isolate it. That\u2019s a huge safety issue\u2014line workers could go to fix a line thinking it\u2019s dead, but it\u2019s still energized by the solar farm. We program all our Digital RMUs to have adjustable directional settings, but we always tell customers to test the bidirectional flow when they\u2019re adding DERs. Last month, we had a fire department call us because a utility line caught fire in a subdivision, and the Outdoor RMU didn\u2019t trip because it couldn\u2019t sense the reverse power from the solar on the rooftops nearby. They fixed it by tweaking the relay\u2019s directional setting for $200, but that\u2019s way better than a line worker getting hurt.<\/p>\n

Now, I know some of you are thinking, \u201cThis sounds like a lot of hoops to jump through. Can\u2019t we just buy an Outdoor RMU and plug it in?\u201d No, not if you want it to work long-term. We see too many customers cut corners on coordination, and it always comes back to bite them. Here\u2019s a quick checklist we give every new client when they get our Outdoor RMUs: 1) Match TCC curves and trip times with upstream\/downstream protection, 2) Verify all voltage ratings (including transient buffer) across the entire node, 3) Confirm comms protocols are compatible (or add a converter if needed), 4) Check physical mounting specs and environmental ratings, 5) Test directional settings if there\u2019s bidirectional power flow. If you go through that list, you\u2019ll avoid 90% of the headaches we deal with on a daily basis.<\/p>\n

At the end of the day, Outdoor RMUs are the workhorses of the outdoor grid, but they don\u2019t work in a vacuum. Coordination isn\u2019t just a technical detail\u2014it\u2019s the difference between a quick fault fix and a 12-hour outage during a heatwave, between safe grid operation and dangerous accidents, between saving money and wasting thousands on replacements. We build our Outdoor RMUs to be tough, but even the best gear fails when it\u2019s paired with the wrong equipment or set up wrong.<\/p>\n

<\/p>\n

If you\u2019re working on a project and have questions about how to coordinate our Outdoor RMUs with your existing power equipment, or if you\u2019re running into issues with a current setup, hit us up to chat. We\u2019re not the kind of supplier that sends a spec sheet and disappears\u2014we\u2019ll walk through the steps, answer the random late-night calls, and help you make sure your grid runs smooth. Because at the end of the day, our job isn\u2019t just selling RMUs\u2014it\u2019s making sure you can keep the lights on for everyone who needs \u2019em.<\/p>\n

Power Quality Treatment<\/a> References:<\/p>\n

    \n
  1. IEEE Std. C37.60-2019, Standard for Outdoor Ring Main Units for AC Power Systems<\/li>\n
  2. North American Electric Reliability Corporation (NERC), \u201cGrid Equipment Coordination Guidelines for Medium-Voltage Distribution Systems,\u201d 2022<\/li>\n
  3. International Electrotechnical Commission (IEC) 61850, Communication Networks and Systems in Substations, 2020 Edition<\/li>\n
  4. U.S. Department of Energy (DOE), \u201cDistributed Energy Resource Interconnection and Grid Coordination Handbook,\u201d 2021<\/li>\n<\/ol>\n
    \n

    Deepwill International Technology Development (Jiangsu) Co., Ltd.<\/a>
    Deepwill International Technology Development (Jiangsu) Co., Ltd. is one of the most professional outdoor rmu manufacturers and suppliers in China, featured by quality products and good price. Please rest assured to buy outdoor rmu for sale here from our factory. Contact us for OEM service.
    Address: No. 3-3 muyang Road, Hanjiang Economic Development Zone, Yangzhou city, Jiangsu Province, China.
    E-mail: deepwill@deep-will.com
    WebSite:     https:\/\/www.deyunelectric.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    Hey everyone, Jamie here from the Outdoor RMU squad. If you\u2019ve ever picked a power distribution … What coordination requirements are there between an Outdoor RMU and other power equipment?<\/span>Read more<\/a><\/p>\n","protected":false},"author":489,"featured_media":3541,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3504],"class_list":["post-3541","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-outdoor-rmu-483f-fac288"],"_links":{"self":[{"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/posts\/3541","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/users\/489"}],"replies":[{"embeddable":true,"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/comments?post=3541"}],"version-history":[{"count":0,"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/posts\/3541\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/posts\/3541"}],"wp:attachment":[{"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/media?parent=3541"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/categories?post=3541"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.eungabiho.com\/blog\/wp-json\/wp\/v2\/tags?post=3541"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}