Downtime Denied: How an External Maintenance Bypass Saves the Day

If your UPS experiences a catastrophic failure or needs to be removed for maintenance, one piece of equipment is crucial so you don’t drop your mission-critical load – an External Maintenance Bypass. Not only do you need to know why it’s important, but also the correct procedure when switching to an external bypass.

When crisis strikes, you’ll be glad you have an External Maintenance BypassWhile an External Maintenance Bypass (EMB) is not mandatory, it does provide your system more flexibility and protection to keep your critical equipment online.  In the event of a catastrophic UPS failure, where your UPS fails to supply power to your mission-critical equipment, an External Maintenance Bypass can save the day. An EMB allows you to reroute power to completely bypass the UPS without interruption to your load.


The other time an EMB is necessary is if you need to service your UPS, and therefore have to completely remove the power from your system. By switching on an EMB, you’re able to service your system without interrupting the load.

Take a look at the different bypass systems and how they work. As you’ll see, getting the right EMB for your facility and knowing how/when to use it can ensure your mission-critical equipment remains online.

Two Types of External Maintenance Bypass Systems

Rotary bypass – This type of EMB is typically used in the smaller to mid-range UPS systems. The system can be designed in two ways, as a “make before break” or as a “break before make” model.

The make before break EMB is exactly as it sounds.  It parallels the external and internal UPS circuit for a short period of time so you don’t break the circuit to the load (to ensure that your load continues to receive power and doesn’t shut down).

The break before make will disconnect or open the circuit for a short period of time, temporarily disrupting power to your load during transfer.

Breaker bypass –  This system also comes in various types/options. The two most common types are “line up” and “matching cabinet” to the existing UPS or an external, wall mounted type.  They can also be specified as 2 or 3 breaker.

Correctly Switching Your UPS to External Bypass

Your UPS can incur damage and other risks if the proper procedures aren’t followed when switching the system to External Bypass.

One thing to keep in mind is that during any type of transfer of power to bypass, the UPS must be on internal static bypass mode. The static bypass ensures that there is no phase shift and the power is aligned. If the power isn’t aligned and there is a phase shift from input of the UPS to the output equipment, damage and potential physical harm could occur.

Some EMB’s come with a green light that indicates it is safe to go into bypass. Others may come with a button that needs to be pushed prior to rotating the bypass knob. This button (if properly installed) will automatically put your UPS system into static bypass so you can’t make a mistake!

Still, some EMB systems don’t have any type of safeguards and rely on the knowledge of the individual to understand what they are doing.

For our purposes I will show you the 3 breaker system and explain the function and procedure to safely bypass (for your system we recommend that you refer to the manufacturer’s instructions). Click on the graphic to enlarge it.Screen-Shot-2014-11-18-at-8.20.02-AM-300x209

The 3 breaker External Maintenance Bypass system has an input breaker (N/C)(Switch 1), an output breaker (N/C)(Switch 3) and a breaker (N/O)(Switch 2), all of which tie the input to the output- bypassing the UPS.

The switching sequence is the most important requirement to ensure that you avoid dropping the load. Additionally, if you don’t have the UPS system on internal bypass, a phase shift (produced by the double conversion of the power internally in the UPS) won’t align with the output power, resulting in a mismatched parallel power.

In order for this system to work properly, you must have the UPS system internally bypassed (static bypass). Once you know that the UPS system has been placed on static bypass, it is safe to begin the switching procedure.

Switch 2, which is normally open, should be switched to the normally closed (N/C) position. At this time, you have all 3 switches in the N/C position. This means that you are (parallel) feeding power through the UPS (static switch) and the bypass switch to your load.

The next step is to change switch 3 to N/O and then switch 1 to N/O. At this point power is completely bypassing the UPS system through switch 2 and delivering power to your load. When you want to take the system out of external maintenance bypass simply reverse the procedure.


Downtime cost calculator



External Maintenance Bypass – Worth the Cost

The cost of a power outage can be very expensive. The 2013 Study on Data Center Outages by Ponemon Institute estimated the cost of an outage at $7,900/minute.

If the UPS fails and the Internal Maintenance Bypass fails, manually bypassing the UPS allows the loads to be supported from the source (either the utility or generator).

The cost of the External Maintenance Bypass is directly proportional to the system size. It is typically about 10% of the overall UPS project.

An External Maintenance Bypass system is critical for keeping your load online in the face of a catastrophe or need to service your UPS. Equip your facility with the right EMB and know the correct operating procedures to ensure your critical equipment remains online.


QPS - Cost of Downtime CalculatorDo you know how much a power outage would cost your company? 

Download this free downtime cost calculator to find out today.



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