Beware Maintenance Bypass Switches. What you need to know.

Beware Maintenance Bypass Switches. What you need to know.

Many installers of hardwired Uninterruptible Power Supplies will insist on an external “wrap-around” Maintenance Bypass Switch. They’re used to allow the UPS to removed from circuit without dropping power to the load for service or maintenance. They’re also handy during the installation phase as this can be wired up prior to a UPS arriving on site allowing the load power distribution to be powered up or tested.

Standard Bypass Panel

The operation is fairly simple. Referring to the picture above the incoming mains supply feeds a switch which supplies power to the UPS input. It also feeds a switch called “Bypass” which is normal operation is open.

The output from the UPS is then connected to another switch (UPS Output) and the output of this feeds the load with a secondary connection feeding the output of the Bypass Switch. Although this sounds confusing if you follow the red lines on the bypass panel above you should be able to figure it out. Although this layout has three switches, other configurations are available with a single switch with usually 3 positions – Normal, UPS Bypass, Bypass.

In the switch configuration shown, there is power feeding the UPS, the UPS output then feeds the load.

When we want to bypass the UPS then this is simple. We just turn the bypass switch from off to on – right? Wrong, very wrong. Because then what happens is the input mains supply is now directly connected to the UPS output which usually results in a destroyed inverter and a popped fuse with the resultant drop in load power that we were trying to avoid in the first instance. Now you may think this is obvious but it does happen as – in the example above – there are no warnings on the panel, and particularly with single switch panels there can be an assumption that there is some sort of safety guard – there is not!

Correct Procedure

In order to correctly bypass the UPS the first thing you need to do is to make sure the UPS is in bypass mode. Sometimes you do this by switching the unit off and it automatically bypasses. Sometimes you need to turn a breaker on the back of the unit.

Once the UPS is in bypass, you can then safely turn the bypass switch on the box from off to on. Then you can switch off the input and output breakers. Load is now provided with power directly from the mains and the UPS has no power feeding it. The UPS can be taken out of circuit for service.

To put the UPS back into normal operation, first switch on the UPS input breaker ensuring the UPS output breaker is still OFF. Make sure power is feeding the UPS and that the UPS is in BYPASS and does not go ONLINE. Then switch on the UPS output breaker, switch the bypass switch from on to off and finally switch the UPS online.


Of course the whole raison d’être for the Maintenance Bypass Switch  is to allow continuity of power whilst the UPS is being maintained and there are a couple of issues that you should be aware of during the bypass procedure.

The first one is obvious. If you get a power disturbance whilst in bypass the UPS is not offering the load any protection. This of course can result in downtime.

When bringing a unit up from bypass there is the pitfall of an in-rush current tripping the supply fuse. This is more prevalent when the unit has an isolation transformer fitted and the electrical supply circuit breaker is not a Type C or D breaker. There’s not really anything you can do if this is the case except switch on and hope.

However a trip has happened to me on several occasions with transformerless products. What transpired is the internal capacitors were fully discharged resulting in a current spike during switch on, which although limited through a start up protection circuit was still high enough to trip a sensitive breaker. For these scenarios a hot tip is before applying power to the UPS, start it up from battery. This charges up the capacitors internally. You can then switch off and quickly reconnect power to start up or allow the unit to synch with the incoming supply.

In any event the bypass procedure must be undertaken with absolute care, and site users must be aware that any bypass operation is not foolproof and there is always the risk of downtime, particularly during power down and power up moments.


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