Most businesses have projected the financial impact of a total system outage, less have valued the likelihood of such events. The risk calculator developed by Static Power, supports businesses to evaluate and compare this likelihood for different power distribution configurations or qualities of components.
MTBF – Mean Time Before Failure
Mean Time Between Failures – MTBF – is the predicted elapsed time between inherent failures of a mechanical or electronic system, during normal system operation.
MTBF can be calculated as the arithmetic mean – average – time between failures of a system.
The term is used for repairable systems, while mean time to failure – MTTF – denotes the expected time to failure for a non-repairable system.
The definition of MTBF depends on the definition of what is considered a failure. For complex, repairable systems, failures are considered to be those out of design conditions which place the system out of service and into a state for repair.
Failures which occur that can be left or maintained in an unrepaired condition, and do not place the system out of service, are not considered failures under this definition. In addition, units that are taken down for routine scheduled maintenance or inventory control are not considered within the definition of failure.
The higher the MTBF, the longer a system is likely to work before failing.
Annualised Failure Rate – AFR
Annualized failure rate – AFR – gives the estimated probability that a device or component will fail during a full year of use.
It is a relation between the mean time between failure – MTBF – and the hours that a number of devices are run per year.
Annualized failure rate – AFR – gives the estimated probability that a device or component will fail during the first full year of use.
Risk of Failure – General
The probability for 2 components to fail in same time will depend on the configuration:
Risk of Failure – STS
ATS/STS function is to switch to an alternate source (B) if the preferred (A) fails.
Knowing;
- the ATS/STS cannot supply the load is the both sources fail (A ∩ B) – parallel case
- And, the ATS/STS has its own MTBF so its own risk of failure (for iSTS 800,000h, 1.1% risk of failure)
- And, the ATS is in series with the 2 sources, series case
The risk of failure of the system [(A and B) xor ATS/STS] is:
STS function is to reduce system risk
Risk of Failure – STS
There are 3 types of ATS/STS on the market:
- Relay: The transfer is done via electromechanics relays; MTBF 100,000h
- Hydrid: The transfer is initiated with thyristors then the relays take the load. The thyristors only sees the full load during the transfer and are not rated for continuous full load; MTBF 200,000h
- Solid-State: Transfer and continuous full load flows through Thyristors. The thyristors are rated for full load and overload; MTBF 800,000h
- The relay type increases the risk of failure of the system
- The risk of not supplying the load is equivalent to the risk of the sources to fail
- The iSTS reduces system risk by 2.4
Choosing the wrong ATS/STS can increase risk of failure
With the same sources, replacing a relay type by an iSTS will reduce risk of failure by 6.7 times
Load or System risk of failure
Single cord: If the STS fails then all loads will not be supplied
Dual cord: If one STS fails then only one load will not be supplied
Financial risk
The different costs to consider in case of a total system outage are:
- Outage average duration (in hours) and cost of lost production ($ per hour)
- Repair average duration – MTTR – (in hours), cost of the repair (in $) and cost of lost production during the repair (in $ per hour)
Cost per outage= [(Outage duration+Repair duration).Cost of lost Production]+Cost to repair
The risk of having a total system outage depends on the risk of failure of the alternate source system and the “risk of failure” of the Main, the both being in parallel.
Main’s outage yearly frequency determines Main’s “risk of failure”
Single, Dual and Dual Redundant Cord
Compared to a “UPS only solution”, addition of one or many iSTS to the system can reduce risk of failure of the load up to 10 times and the system up to 72 times.
iSTS reduce load, system and financial risk
Choosing a Critical Power Distribution Configuration
e.g: With an average of 8 outages of 1 hour per year, 4 hours to repair and no cost to repair
If the cost of a full system failure is $20k/h (by 1h during the outage + 4h to repair) then the yearly financial losses for a “UPS only” solution is $83k, $17,000 for a single cord (divided by 5) and $1,300 for dual cord redundant system (divided by 64)
The choice of the most adapted Critical Power Distribution configuration is driven by the financial risks faced by the Business.
Financial risk is the product of:
1. Mains outage frequency
2. Financial losses in case of full shutdown
3. System risk of failure
While the first 2 points are not under Businesses’ control, System’s risk can be mitigated by adequat design and high quality Static Transfer Switches