Resources / Static Transfer Switch Bypass Overview
Static transfer switch bypass overview
Every critical power device in a facility can be bypassed for maintenance: the UPS, the generator ATS, even the main switchboards. A transfer switch at the rack deserves the same. Without a bypass, the response to any alarm is to drop the load.
Bypass and bypass isolation
A maintenance bypass is a manually operated path around the electronics, from one source straight to the load. Bypass isolation goes a step further. It also disconnects the electronics from both sources and from the load, so the module that holds the logic and the SCRs can be withdrawn safely while the load keeps running.
On the DCC-II and DCC-III the bypass is built into the outer enclosure at no extra cost, and the plug-in electronic module carries everything else. Before the bypass closes, the selector commands the module onto the source you are bypassing to; if it cannot get there, a Do Not Bypass alarm stops you. Optional mechanical interlocks (Interlock I and II) also lock the module in place until the selector reaches a bypass-isolated position. The rack and cabinet DCC-4 models include a maintenance bypass, and the rack-mount versions are hot-swappable.
Walk through the selector
Step through the seven positions to see which path carries the load and when the electronic module is safe to withdraw.
Sequence shown for the DCC-II and DCC-III. Always follow the operating manual for your unit.
Why the order matters
The bypass closes onto the same source that is already feeding the load through the electronics. That is why the selector first commands the module to a source and only then closes the bypass to it: the two paths are in parallel for a moment, on one source, and the load never sees a gap. At no point are the two sources connected to each other.
While the switch is bypassed, the load is on a single source with no automatic transfer. Keep that window short, and choose the healthier source before you start.
Questions to ask any vendor
- Is there a bypass at all? Most 1U relay switches have none.
- Can the electronics be removed with the load running?
- Does it stop you bypassing to a source it cannot reach, and are interlocks available against the wrong sequence?
- Where are the fuses, and can they be replaced from outside the unit in a minute or two?
- Does an alarm require a shutdown to investigate?
Questions engineers ask
Is the load protected while the switch is in bypass?
It is powered, from one source, without automatic transfer. That is the same trade every bypass makes. Plan the work, pick the better source and keep it brief.
Do relay-based rack switches have a bypass?
Most do not. When one alarms, the only option is to schedule an outage for the load it feeds.
How long does a module swap take?
A few minutes. The module plugs in, so there is no wiring to disturb.
Put it to work
Narrow the range to your voltage, current and mounting, or send us your one-line diagram and talk to an engineer.