In short. Dual-corded equipment has two power inputs and can run from either, so it is the natural fit for A/B distribution. Single-corded equipment has one input and needs a transfer device to gain redundancy, which becomes a single point of failure itself. Know which loads are which and plan the connection of each.
Dual-cord loads
Servers, network switches and storage systems commonly have two power supplies, each connected to a separate path. If one supply or path fails, the other continues. Load usually shares between the two supplies in normal operation, though some equipment uses one as active and one as standby.
Single-cord loads
Some devices, such as older equipment, appliances or some instruments, have only one cord. Options:
- Connect to one path and accept the lower resilience.
- Connect through a static transfer switch (STS) fed from both paths. See static transfer switches.
- Use a rack-level automatic transfer switch.
- Replace or modify the equipment where practical.
Comparison
| Aspect | Dual-cord | Single-cord with STS |
|---|---|---|
| Resilience | Both paths independent | Limited by the transfer switch |
| Transfer time | None; both supplies live | Typically a few milliseconds |
| Added cost | None for the equipment | STS cost and space |
| Added failure points | Few | The STS and its control |
Common faults in dual-cord systems
- Both cords on one path: a wiring error that defeats redundancy, often found only in an outage test.
- Uneven sharing: one supply carries most of the load, so failover doubles its current.
- Overloaded paths: see the capacity rule in A/B distribution.
- Different phase rotation or voltage on paths: some equipment misbehaves.
- Surge currents at failover: checking supply capability helps.
Verifying real redundancy
- Label every cord by path at both ends.
- Audit each device for both cords and path assignment.
- Switch each path off in turn and confirm all devices remain up.
- Measure load per path before and after the test.
Worked illustration
A rack has ten dual-corded servers and two single-corded network appliances. The servers are connected correctly to A and B. The appliances both sit on path A. When path A is maintained, the appliances lose power even though the servers survive, so the network drops. A rack STS fed from A and B for the two appliances fixes the exposure, at the cost of one more component to manage.
Phase and polarity checks
Where a single-corded load is moved between paths by an STS, confirm both paths have the same phase rotation, voltage and, for three-phase loads, phase sequence. A mismatch can cause equipment to reverse, trip or run at the wrong voltage after transfer. Where separate transformers feed the paths, check their vector groups and tap settings.
Policy suggestions
- Require dual-corded equipment for new critical loads where possible.
- Keep a register of single-corded devices and how each is protected.
- Repeat the redundancy audit after each change.
Dual-cord versus single-cord
| Type | Power supplies | Behaviour on one path failure |
|---|---|---|
| Dual-cord | Two independent supplies, each sized for the load | Load shifts to the other supply with no interruption |
| Single-cord | One supply | Loses power if its path fails |
Making single-cord loads resilient
- Rack-level STS: a static transfer switch takes two feeds and delivers one output, switching in about 4 to 8 ms.
- Replace the power supply with a dual-input unit where the equipment allows.
- Locate single-cord equipment on the more reliable path, accepting a defined risk.
Check items
- Is the equipment’s power supply tolerant of the STS transfer time? Most are.
- Do both supplies share the same phase and sequence?
- Is the STS itself a single point of failure for those loads?
Inventory
List every critical device, its cord type, the paths it draws from and the consequence if one path fails. Update when equipment changes, because a device installed by habit onto one path can undermine the architecture.
See static transfer switch systems and A/B distribution.
What to send us
Send an equipment list showing the number of power supplies per device. Send your application details. See also critical-power bus architecture.
Questions engineers ask
Do I need an STS for single-corded loads?
Only if the load needs more resilience than one path gives. The STS gives the load two sources but adds its own failure point.
How fast does an STS transfer?
Usually within a few milliseconds, fast enough for most electronic equipment. Check the equipment’s hold-up time.
How do I find wiring errors?
Label cords and test by switching each path off in turn while watching all devices.
Why is load sharing between supplies important?
If one supply carries most of the load, failover doubles its current and may overload it.
This guide gives general principles. Confirm equipment specifications from manufacturers.