Static Transfer Switches (STS) are a type of electrical switch that can perform automatic and manual transfers of electric loads between two AC power sources.
STS monitor the incoming sources and output and transfer the load to an alternate power source when failure or degradation is detected.
i-STS switches use solid-state thyristors, also known as silicon-controlled rectifiers (SCRs), to perform transfers that are fast enough that the break in current is not recognised by the load. Transfers are typically less than 1 millisecond and even under worst-case conditions only up to 5 milliseconds (1/4 cycle) which is suitable for even most sensitive equipment.
Visit our i-STS Technology page to learn more about the benefits and features of using i-STS switches.
The silicon-controlled rectifiers (SCRs)/thyristors are organised in pairs in a back-to-back configuration, one for each source with the output common to supply the critical load. A control signal is sent to the thyristor to either allow or inhibit conduction and switching occurs almost instantaneously.
The control electronics continuously monitors the power to the critical load and the incoming supplies. When degradation or failure is detected the STS initiates a transfer to the alternate source. The STS can detect failures in just a fraction of a cycle (typically 1-3 msec) then transfer to the alternate source within a quarter of a cycle.
The STS will not transfer load faults to the alternate source even if this results in a loss of power to the critical load which prevents damage to upstream sources.
i-STS switches will detect the frequency of both incoming sources and automatically adjust the timing of transfers to optimise the transparency of transfer. This is done by inserting break longer break times for asynchronous transfer and transferring only at the zero point cross-over.
Automatic Transfer Switches (ATS) and Relay-Based Transfer Switches
Automatic transfer switches (ATS) and other relay-based transfer switches use electromechanical devices to switch an electrical load between two power sources. Relay-based switches are sometimes preferred in smaller de-centralised or "point of distribution" installations because they are less expensive and suit installations that employ residual current devices (RCDs) and Earth-leakage circuit breaker (ELCB).
There are drawbacks that equate to a lower reliability and therefor a higher probability of system critical power failure.
Relay-based switches respond to supply failures but wane in the detection of brown-out conditions. The conditions that will trigger a transfer are somewhat uncertain and the transfer timing can be erratic. Transfers may take place at any point in the AC waveform generating dangerous voltage spikes causing arcing and damage to components. Because relay-based switches only monitor voltage they may send distorted power to the load.
Electromechanical switches are have the potential to perform slow transfers in the events of power loss or low supply with break times ranging from 12-25 msec which is long enough to affect some equipment.
Compared to the SCR/thyristors in STS, the electromechanical devices in relay-based switches are physically larger with lower current capacity. Electromechanical devices use moving parts and as such the performance will decline over time and until eventually wearing out completely. Transfers that generate dangerous voltage spikes and arcing can cause the mechanical switching components to become welded closed or blown open, rendering the unit and ineffective and irreparable.
Static Transfer Switch (STS)
STS employ silicon controlled rectifiers (SCR)/thyristors which are ideal power switching devices. Thyristors have no moving parts and will not suffer any wear from switching so there is no practical limit to the amount of transfers that can be performed. Transfer time is typically less than 1 millisecond and under the worst case conditions up to only 5 milliseconds (or a quarter cycle) which is suitable for the most sensitive equipment. Furthermore, thyristors are very capable of withstanding large fault currents.
Relay switches generally consist of an enclosure housing the electromechanical components, while STS are more compact devices with internal electronics to optimise the transparency of transfers and provide a high level interface for remote control and monitoring.
In 1-pole transfer switches the neutrals do not switch and are common to both sources and the output. There is only one active current path and one neutral return path.
With 2 poles, the neutral switches as well as the active conductors which makes it possible to steer the neutral currents back to their sources and adjacent phase conductors.
This reduces the impedance resulting in less voltage distortion and lower voltage drop at the load. It also ensures that neutral conductors are not inadvertently overloaded as a result of stray neutral currents from other loads (which could cause neutral overloading within the site).
Similar to the scenario of a 2nd pole for single-phase switch, the 4th pole reduces the voltage distortion to the load.
By switching the neutral as well as the active conductors, it is possible to steer the currents back to their sources and adjacent phase conductors thereby reducing the impedance resulting less voltage distortion and lower voltage drop at the load.
3-wire distribution systems will not benefit from a 4th pole in an transfer switch. If a distribution system has Multiple Earthed Neutral (MEN) or neutral bonding points and the cable lengths are short then a 3-pole transfer switch will be suitable.
If a distribution system does not employ a Multiple Earthed Neutral (MEN) system for interconnected continuity of Neutral to Earth potential bonding, or if it employs Neutral Isolation (e.g. 4-pole isolation or protection circuit breakers), then there will be a benefit to use a 4-pole transfer switch.
4-pole STS will not generate dangerous voltages because the neutral switching is a make-before-break (overlapping) process and stays on until all of the currents in the previous conducting phases have ceased (approximately 15 msec).
Smaller units suited to point of distribution have fuses for protecting the load against faults, generally the fuses are rated to 100 Amps. The internal fuses are for final STS protection only. We would expect that the downstream fuses in the faulty equipment will always clear first. Thus loss of output due to internal STS failure is not likely to occur except in the most arduous operating conditions.
Larger models don't have internal protective fuses because the switching elements and internal bracing are generously overrated to cope with external short circuits without failure, damage or loss of output.
For safety reasons there needs to be some protection between the source and the load. This protection is graded to limit the fault currents to below the specified fault rating (kA rating) such that the protective device opens the circuit within 1 cycle. Most circuit breakers will enable settings to be selected to easily achieve this.
All i-STS Static Transfer Switches support remote interrogation and control. There is no proprietary software that is required to access or display any of the system information.
All i-STS models have an Ethernet RJ45 port to connect to a LAN, allowing the internet browser interface to be used for remote control of transfers and access to all user settings, variables and events.
Network Time Protocol (NTP)
Connecting via Ethernet enables synchronisation of the STS system clock using Network Time Protocol (NTP).
Modbus TCP/IP is available via Ethernet. Modbus via RS422 or RS485 multi-drop is available with a third-party hardware adapter. This enables the extraction of variables, status, alarms and event histories as well as control of transfers between sources.
Uses modified UPS MIB.
All models that are connect to a LAN via Ethernet will send an email to the user when a fault condition triggers the alarm.
All models have voltage free discrete hardware contacts that can be used for local Building Management Systems (BMS). All units have at least 5 pre-defined voltage free change-over contacts and at least two control inputs, with the exception of the i-STS Model A1 which has one contact for General Alarm.
Emergency Power Off (WPO)
Remote Emergency Power Off (WPO) is an optional add-on.
Preferred Source Selection
A preferred source can be selected, either Supply 1, Supply 2 or neither. The STS will connect the load to the preferred source whenever it is available. The preferred source setting can be changed remotely through the web browser interface, however certain models also have a physical switch to select the preferred source.
Override Source Selection Switch
Certain models have a physical override source selection switch. Setting the override switch to Supply 1 or Supply 2 will force the STS to keep the load connected to that supply even if faults are detected. This switch overrides all other settings and also inhibits all manual and automatic transfers.
All models have Thyristor Short Circuit (S/C) and Open Circuit (O/C) Detection protection in case of the rare event of SCR failure.
In the event of SCR failure on a source, the STS will be be put into a safe state to protect the load. When S/C or O/C is detected on the inactive source, transfers to that source are inhibited.When S/C or O/C is detected on the active source, the load is transferred to the alternate source and further transfers are inhibited until the condition has been addressed. The alarm condition is triggered and will remain on until repair can be made.
In many cases the detection of these states may also be accompanied by automatic physical isolation by the tripping of circuit breakers or other electromechanical switches within the STS.
All i-STS models either come standard with or have the option of being fitted with incoming source isolator switches which can be manually operated.
Most models have the provision for remotely controlled Emergency Power Off (EPO) via a voltage free contact. This is a custom option which is not selectable in our product pages so please contact us when requesting a quote.
All i-STS units have universal frequency tolerances which allow them to be installed anywhere with 50 and 60 Hz. The STS will determine the operating frequency at start-up.
The voltage rating of an STS requires specific hardware and firmware so it must be determined when placing an order. The available standard/nominal operating voltage rating differs between i-STS models so you will need to check the specifications of each model, or contact us for more information.
There is scope to customise i-STS models with various types of inlet and outlet plug types, leads, cable entry points and terminals. Please note that any changes beyond the standard options of an i-STS model means that the STS will be classified as a custom model.
Rack mount models
The standard inlet types include IEC C20 and terminals.
The standard outlet types include IEC C13, C19, NEMA 5-20R, Aust GPO and terminals.
There is scope to use non-standard inlet and outlet types.
The standard inlet and outlet types use terminals and either top or bottom cable entry.
There is scope to have different cable entry points and non-standard terminal types.
Please contact us to discuss using any non-standard inlet and outlet types and cable entry points.
i-STS products have an IP rating listed in the technical specifications table. The IP CodeInternational Protection Marking is often interpreted as Ingress Protection Marking, is a two digit code that classifies and rates equipment for the degree of protection provided against intrusion.
The first digit classifies the degree of protection provided against solid objects (dust, dirt, accidental contact etc.) and the second digit is for liquid ingress protection.
1st Digit - Protection Against Solid Objects
0 - No Protection.
1 - Protected against solid objects greater than 50mm such as hands.
2 - Protected against solid objects greater than 12.5mm such as fingers.
3 - Protected against solid objects greater than 2.5mm such as tools and thick wires.
4 - Protected against solid objects greater than 1mm such as wires, screws, insects.
5 - Protected against dust sufficiently to prevent interference with operation of the equipment.
6 - No ingress of dust.
2nd Digit - Protection Against Liquids
0 - No Protection.
1 - Protected against against dripping water.
2 - Protected against water drops at 15° angle.
3 - Protected against water spray at 60° angle.
4 - Protected against water splashing at angle angle.
5 - Protected against water jets from any angle.
6 - Protected against powerful water jets.
7 - Protected against immersion in water up to 1m depth for 30 minutes.
8 - Protected against immersion in water beyond 1m depth.
Levels and tiers are a way of describing the requirements for data centre infrastructure.
The Telecommunications Industry Association, a trade association accredited by ANSI (American National Standards Institute) defined four "Levels" of data centers, however more commonly used are the "Tier Standards" defined by Uptime Institute, based in Seattle, Washington.
For Uptime Institute's standards model, the Tiers are progressive so that the higher the level, the greater the expected availability of data processing, with each Tier incorporating the requirements of all the lower Tiers. It is worth noting that in 2009, all reference to “expected downtime per year” and availability predictions was removed from the Tier Standard.
Topology diagrams of Tier 2-4 data centres will often show an ATS (Automatic Transfer Switch) connected to generators or UPS, which is where a STS (Static Transfer Switch) can be utilised.
Uptime Institute Tier Level Characteristics:
Tier 1 - Generally utilised by small businesses
Single non-redundant distribution path serving the critical loads
Non-redundant critical capacity components
Basic UPS system with one path for power delivery and no redundancy
Off-line maintenance for many components
Tier 2 - Partial redundancy
Meets all Tier 1 requirements
Redundant critical capacity components
Basic UPS system with one path for power delivery and some redundant components
Partial redundancy in power and cooling
Off-line maintenance for some components
Tier 3 - Generally utilised by larger businesses
Meets all Tier 2 requirements
Single UPS system with redundancy and dual/multiple active power delivery paths
Concurrent maintenance possible with critical operations on generator or alternate path
Multiple independent distinct distribution paths serving the IT equipment critical loads
All IT equipment must be dual-powered provided with two redundant, distinct UPS feeders
Single corded IT devices use a Transfer Switch with two UPS feeders.
N+1 fault tolerant providing at least 72 hour power outage protection
Tier 4 - Generally utilised by enterprise corporations
Meets all Tier 3 requirements
Multiple independent distinct and active distribution paths serving the critical loads
Critical systems must be able to autonomously provide N capacity to the critical loads after any single fault or failure
Continuous Cooling is required for IT and UPS systems.
The current rating of an STS will depend the current demand of the sum of all of your connected equipment. The most accurate way to measure this is to undertake a measurement audit during normal business operation. Certainty is improved with continuous hourly logging over 24 hours for a period of one to two months, especially if there are times of varying demands.
STS should be rated by their maximum RMS phase current (not generally by kVA and KW) and any potential growth must be considered.
Measure and monitor load neutral currents because cumulatively these may be greater than the phase currents. In cases that the neutral currents are greater than the phase currents when a 4-pole STS is under consideration, the best solution is to chose an overrated STS.
This is because the 4th pole in any series isolator or maintenance bypass facility should have the same current rating twice that of the phase conductors. Hence an STS with a phase current rating of 250 Amp and 2 times neutral rating will actually need to be rated to 500 Amp. This only applies for 4-pole STS, and not for 3-pole STS with a common neutral.
Voltage Rating To determine the specifications of an STS at the time of ordering, you will need to provide your nominal voltage, frequency and any significant specific electrical variations (e.g. must operate from generator with a freq variation of X, or a Voltage variation of Y, or a THDV of Z etc).
Some abnormalities in your system may have no affect on implementation although they must be considered. For example, an STS may be supplied by 2 UPSs with similar rating that originate from different manufacturers and so they do not have the same topology or performance. In these cases the STS operating parameters should be adjusted at the time of manufacture to provide better protection for the load and the sources.
STS can be implemented generally in two ways, centralised and de-centralised.
De-centralised distribution uses small STS installed at point of distribution/point of usage. Typically these STS are rated from 10-120 Amps and can utilise 19-inch IT equipment racks. i-STS Models A1, B1, B2, B3 & W suit this type of application.
De-centralised distribution generally provides greater overall system reliability because failures of one aspect of the distribution system will not affect of the mission critical infrastructure as a whole. There is also greater protection against accidental upstream switching and cascading tripping of breakers.
Centralised distribution is where a large capacity STS delivers power from an uninterruptible power supply (UPS) to the load. i-STS Models C3, H, K & G suit this type of application.
These STS are typically rated from 100-1250 Amps and need to be capable of withstanding large fault currents because they are usually placed in, near or adjacent to the electrical power reticulation infrastructure. UPSs may be located on a different floor to the load and distribution of power is via sub boards on different floor levels.
In the case of dual-cord distribution, two UPSs and STS are employed to deliver two feeders throughout a building. A dual-cord centralised distribution scheme has greater reliability and a higher Mean time between failures (MTBF).
Other schemes may use a large STS to distribute the main feeders of two UPSs up a building close to the critical loads, then use wall mount or 19-inch rack STSs at the point of distribution to supply critical non-dual cord equipment power. This provides single-cord equipment with almost the same reliability as the dual-cord equipment.
Generally the smaller models up to use 100 Amp internal fuses while the larger models above 63 Amps incorporate current limiting circuit breakers.
When larger STS are implemented, it makes for simpler discrimination when it comes to downstream or load faults if the STS plays no role in the protection scheme, which is best left to the downstream and upstream devices.
To achieve discrimination successfully, the maximum current of the breakers upstream of the STS should match the current capacity of the STS (generally 20 or 35 kA, up to 65 kA for 1 cycle depending on the STS).
Normally this is a case of adjusting the dial on the circuit breaker according to the manufacturers’ tripping curves but you will also need to know your maximum source fault current, length and size of cable between the source and the STS. Please contact us for help when calibrating your upstream circuit breakers.