Power Protection for Oxygen Concentrators; the why, what and how

The electrical requirements of oxygen concentrators most frequently used in low resource settings (such as the AirSep Elite and DeVilbiss 525KS) is a constant 230VAC at 50 Hz. Power systems are inherently dynamic entities that are expected to fluctuate during normal operation. However, in low-resource settings (LRS) fluctuations can be greater in magnitude and for greater durations than the thresholds electrical equipment can handle. In fact, poor quality mains power supply is known to be one of the primary causes of malfunction and failure of medical equipment, especially oxygen concentrators in low- and middle-income countries (LMICs). A summary of the most frequently observed power quality issues and their impact on oxygen concentrators and oxygen delivery is given below. We’ve provided a glossary of terms at the end of the article, so if you are unfamiliar with any terms used check this out.

Outages

We define an outage as when the electrical power goes below 50% of the nominal line voltage for more than 10 milliseconds. Short duration outages normally happen when switching between grid-electricity and backup power. When the power returns there can be damaging voltage spikes (described below). Long duration outages (minutes to hours) prevent the use of an oxygen concentrator and either backup stored power or a reservoir of stored oxygen is required to ensure an uninterrupted oxygen supply. Long duration outages are not uncommon in LMICs and many health facilities experience them.

Voltage Sags

Voltage sags are reductions in the mains voltage (typically defined as less than 90% nominal RMS voltage) which last longer than 10 milliseconds. They can be caused by large electrical loads being switched on and overloading the grid or if the grid capacity is reduced or altered, and are commonly observed by incandescent lighting appearing to dim. Sags that last for more than 10 seconds can cause significant damage to oxygen concentrators, in LMICs they can last hours to days. A study on power quality in low resource settings (2019) reported periods days-long transient undervoltage. During a voltage sag the motor (driving the compressor of the oxygen concentrator) might stop running. Not only does the oxygen stop flowing, but with the current still being drawn by the motor, the coils overheat, leading to the seals of the compressor failing and over time the motor coils might burnout. Even if failure does not occur straight away, the life of the compressor is reduced.

Voltage Surges/Swells

Voltage surges/swells are when the mains voltage increases beyond 110% of the nominal voltage, (e.g. for a 230V supply, this would be greater than 253V) and lasts for less than 1 continuous minute. They are different to voltage spikes, discussed next, on account of their longevity, voltage surges last longer than spikes. There are internal and external sources of a voltage surge. Internal sources are local changes in electrical conditions that cause the voltage rise and are frequently caused by large loads (such as compressors) being turned off suddenly. External sources are the result of grid related issues, for example, a large number of air conditioning units all turning off en masse, or it might be caused by a lighting strike to an overhead power distribution cable. Surge events are often observed by incandescent lights appearing to brighten. Severe voltage increases can cause equipment to draw too much power, which can lead to damage of the microcontroller as well as the compressor.

Overvoltage

When the voltage remains high for an extended period of time, greater than 1 continuous minute. The study (referenced above) on power quality in low resource settings devices recorded a high level of locations sustained (greater than 1 minute) overvoltage of at least 350 V, with many lasting up to 5 minutes and a number lasting more than 2 hours. Other locations experienced periods with levels greater than 415V. These levels of voltage for such extended periods can lead to high currents and the permanent damage of devices. This data highlights the importance of investing in adequate protection for Oxygen Concentrators, especially considering that even a single event can result in equipment being damaged, potentially effecting level of care a health facility can offer.

Voltage Spikes

Voltage spikes are transient voltage disruptions lasting less than 10 microseconds and can be a sudden increase in peak voltage — this might be around 500V, but could be up to 1,000V (although this is much rarer). These are short lived increases in voltage, often caused by industrial equipment such as electric welding machines, lightning strikes, or faults in the power supply system such as a transformer. Another common cause might be a poor quality/malfunctioning generator, which might be used as a backup, and frequently occur as result of fuel issues with the system. Large spikes can be damaging to sensitive electronics such as microcontrollers and circuit boards. A voltage spike can produce a corresponding increase in current in the motor coils. Damage is proportional the amount of energy (joules) contained in the spike. Large amounts of energy, without sufficient projection, can lead to damage to logic circuit boards — this is a common mode of failure observed in oxygen concentrators.

Frequency fluctuations

Frequency fluctuations are fluctuations in the cycling rate of the alternating current (nominally 50 Hertz (Hz) or cycles per second). Changes in frequency can affect electric motor rotational speed. Depending on the extent of the fluctuations, this can affect the performance of an oxygen concentrator. In the referenced study a small percentrage of devices experienced extended interuptions due to frequencies maintained outside of the 45 Hz to 55 Hz range. Although not as common as electrical voltage related events, frequency excursions do occur and adequate protection should be considered (e.g. when considering the specification of the voltage stabiliser used).

Oxygen concentrators need protection

Failing to protect concentrators from being damaged by poor power quality frequently results in damaged systems ending up in oxygen concentrator graveyards. It is possible to protect against most power damage through the use of readily available solutions. In the next section we are going to overview some of the considerations and options available to allow you to be able to make the best choice for your situation.

The questions to ask in selecting a power protection solution

There are a number of options for protecting an oxygen concentrator from poor power quality. What you select will depend on numerous factors including how poor the electrical supply is, how often you expect a power outage, how long the outage normally lasts for, and the power supply options you have available. It is important to ask yourself the following questions:

• How critical is your requirement for oxygen?
• What options do you have for your source of power? (e.g. grid-connected electricity, off-grid renewables) Is the amount of power sufficient? Remember that oxygen concentrators use inductive motors and can have starting power requirements that are 5–7 times the average running power. The power system must be oversized to match these startup power requirements.
• What is the quality of power? What is the frequency of voltage spikes and sags? How consistent is the supply frequency? (for example, do you notice incandescent lights frequently dimming? This can be an indicator of a power drop)
• Do you have a backup generator that would power the oxygen concentrator? How well is this maintained? Is fuel always available?
• How likely is the power supply to fail? (blackout/brownout) How often does this occur? (How often do you notice the power switches off? For how long a period will it typically stay off?)
• What will be the impact of not investing in power quality protection?

Asking yourself these questions, and finding out as much information as possible about the power supply on which you rely will help you make the best decision. It is important to consider the lifetime cost of your concentrator — sufficient protection will give your system a much longer life and save you money from costly repairs, resulting in a lower cost of ownership.

How can I be more aware of the power quality in my area?

To better understand the quality of electricity in your area, it is helpful to collect data on outages, voltage sags, voltage surges, voltage spikes, and frequency deviations. This can be done in various ways and you may choose to employ several different approaches before making a decision on how you need to protect your oxygen concentrator.

1. Contact Electricity Providers: electricity providers often have data on power quality and might be willing to provide this. This would be a low resource and simple way to obtain information on power quality, however, there may not be a willingness to share data. Mining and other large industries will also track power statistics and may be willing to share.
2. Logging when back up power is used: logging when a generator/battery backup is used and its duration provides good insight on the extent of power outages/voltage sags in your region. It does not however capture data on power quality. This data is best collected digitally with a data logger.
3. Survey of people working in a health facility: create a simple survey to allow people to give their experience of power quality e.g. power availability, experience of using the power supply for different equipment, how often the lights dim/brighten. Although the responses can be subjective, it provides an insight to peoples real life experience, which can help in decision making.
4. Spot check using a power quality meter: a logging multimeter can be used to, over a period of time, to monitor the voltage in a cost-effective way to help identify quality issues. Although make sure you understand the risks associated with doing this and have the appropriate training, do not hesitate to contact a qualified electrician for support. There are also specialist devices which plug into electrical outlets and are able to collect data on power quality (often known as power loggers). They track when events such as blackouts, surges, etc. occur. Data can be downloaded onto a computer analysed to spot key trends over time. N.B. Using devices like this can be time and resource intensive as data can be collected from individual sample points, but it does provide good insights to complement observations (e.g. from a survey). However, power quality can vary over time (e.g. issues could be seasonal) and so when the observations are done could affect the observations and this might not be representative. (Click here for an example device)
5. Continuous remote monitoring: more sophisticated monitors can be installed which collect data and send via GSM to a cloud server. These might collect data on power quality over long periods of time and a more accurate view of seasonal patterns in power quality.

Once you have an idea of power quality in your area and how critical oxygen concentrators are in providing the supply, you are ready to select the technologies for protection of your system.

Overview of protection technologies

The technologies described below are currently the most common approaches to protect AC powered concentrators.

Under Voltage Protection: a simple Way to Protect against Voltage Sags

When the voltage drops and there is insufficient power to continue to rotate the shaft, this can result in overheating of the coils. Ultimately this leads to damage of the compressor seals, and a shortening of the life of the concentrator. A simple way to protect a concentrator against this failure mode is to make use of an under voltage protection device. The protection device sits between the concentrator and the mains supply. When the voltage drops below a pre-set level, which would cause damage, the oxygen concentrator is disconnected in order to protect it. These devices are relatively low cost, simple and aimed at expensive home appliances such as fridges and air conditioners.

Over Voltage protection

Similar to under voltage protection, over voltage protectors would cut off the concentrator when the voltage goes above a set threshold. These are normally bundled together with under voltage protection devices.

Start-up delay

It is common to have a large surge when power returns after a power cut. Functionality to prevent damage is normally built into the low-voltage cut off device. This device causes a short delay in reconnecting the mains power to the concentrator when the power comes back on (e.g. after a blackout), because it is during this start-up period when surges and spikes are known to commonly occur. A delay also ensures that the appliance is not switched on-off repeatedly during fluctuations.

Sollatek produce suitable devices which combine the above protection features.

Voltage Spike Protection

There are various types of protectors for large voltage spikes, some divert the current to earth shorting the current, and others dissipate as heat. Most devices will typically trigger at a set voltage of around 3 to 4 times the nominal mains voltage. Shorting to earth is typically done by spark gaps, discharge tubes, zener-type semiconductors and metal oxide varistors (MOV). MOV are a very common product used to protect oxygen concentrators and other equipment. A MOV contains a semiconductor material which begins to conduct at a certain voltage threshold, allowing power to be either diverted to ground and/or converted to and dissipated as heat.

These devices have several different common specifications which include: the let-through voltage, which is the voltage thresholds at which the device activates; the joule rating, which gives the amount of energy that the suppressor can absorb before failure (a higher joule rating indicates that the unit can absorb a greater number of spikes per unit of time/more severe spikes); response time, the time taken for response to a voltage spike, faster being better. MOVs have a finite life and degrade every time they activate until no protection is offered and the device needs to be replaced. Some modern MOVs have an LED to indicate that they are still functioning.

Since spikes tend to last microseconds, the temperature rise of these devices is minimal. However, if the spike is large enough or lasts long enough, the MOV can be destroyed and power lines melted. For these reasons, it is important to be aware that surge suppressors are not able to completely protect equipment from all events, and direct lightning strikes to power lines, for example, are likely to destroy the protection device. It is therefore safest to unplug equipment during a lightning storm.

Finally, although we are discussing protection at the device level, it should be noted that the protection can be installed at a sub-system level, at the power panel, for example, protecting a whole office/hospital. This might be something that you consider if it is more than oxygen concentrators that you are seeking to protect. Sollatek make such a device which offers additional protection against lightning strikes, where this risk is particularly high this device is available.

Other spike protection technology exists, for example it includes transient voltage suppression diodes (TVS diodes). This is a type of zener diode which can limit voltage spikes in AC power systems. It works in a similar way to an MOV and begins conducting at a given voltage threshold. However, it acts much faster (within picoseconds), but it only has a relatively low energy-absorbing capacity. The maximum voltage range this device can be used is around twice the nominal mains voltage, and will last a long time if used within its limits. Outside of this, with voltage spikes that are much larger, the diode may permanently fail and cause a short circuit. For this reason, TVS diodes are best suited to regions where there are frequent but small voltage spikes. These are very low cost components which could be incorporated more widely into oxygen concentrators.

Devices such as the AVS13 by Sollatek, have appliance level surge projection built in.

Voltage Stabilisers

Voltage stabilisers step up and step down the voltage in order to provide a consistent 230V supply to the concentrator, specifically addressing the voltage sags and surges. Whereas the most basic form of protection cuts off the power to the concentrator when the voltage goes above or below the safe operating range, stabilisation can be used to extend the usable voltage range in which the device can operate — e.g. certain locations will experience regular large deviations in voltage that would benefit from stabilisation. Extended range voltage stablizers would stabiliser an input of 110V to 278V to the nominal voltage, which would enable utilizing the vast majority of the voltage levels in practice. Above 278V the stabliser would cut off supply to the concentrator — and enter a self preservation mode. This is also true for when voltage goes below 110V — the stabiliser would shut off the concentrator in order to protect it. What form of protection is best for you will depend on the power quality in your area and how much additional usable power you would get from the use of a stabiliser.

In selecting a stabiliser, the large current required by the air compressors during start-up (which can be ~5 times the operational current), should be considered. This can cause many off-the-shelf voltage stabilisers to fail. Some voltage stabilizers may have some inbuilt surge suppression capabilities as well. Examples of suitable devices, which also incorporate many of the other protection features described above, include Sollatek’s SVS04–22, HVS-1000 or an extended version with a wider voltage input range SVS04–22E. Sollatek also have an OEM range which can be integrated into the design of the oxygen concentrator, it is known as their FSP range. A suitable device to protect concentrators would be the FSP04. It is also possible to protect the whole building, e.g. with multiple medical devices on the same circuit, using the SVS20. See here for a overview of the full range of Sollateks’s SVS devices to help you assess the most suitable for your needs.

UPS devices

Uninterruptible Power Supplies (UPS) provide uninterrupted power to devices when the primary power is disrupted for a short duration (typically less than 1 hour) and can provide temporary power stabilisation. It is often used to bridge the time between the grid power supply going down and a generator starting up. UPS devices automatically determine whether to draw power from batteries or from the grid. They also typically contain circuitry to detect when there are problems with grid power (e.g., over voltage, under voltage, outages), manage this (such as in built voltage stabilizers) and they might also contain surge protectors.

There are three main types of UPS device:

• Off-line UPS devices are the most basic. During normal operation they get their power from the mains grid, but once it senses that the power has gone beyond acceptable limits or fails, it switches to the “offline” battery, where it will then convert power from DC to AC via a built-in inverter. The disadvantage of this configuration is that there is a switching time, known as the transfer time, between the power going off and switching to battery power
• Online UPS devices differ from the offline version by always converting all power from AC-DC and then DC to AC. This means there will be no transfer time, and the quality of the power will be better. However, the double conversion will lead to a loss in efficiency. An online UPS delivers continuous, high-quality AC power to equipment with no break when transferring to battery, protecting equipment from virtually all power disturbances due to blackouts, brownouts, sags, surges or noise interference
• Line interactive UPS devices operate in similar way to off-line UPS but with the addition of a built-in Automatic Voltage Stabiliser (AVS). This ensures the output voltage remains within a predefined window regardless of variations on the mains input supply. This enables line interactive UPS to provide protection against power sags and surges better than offline UPS devices. When the mains power supply fails or fluctuates outside of the acceptable threshold, the load is transferred (via a relay) to battery power via an inverter. Line-interactive UPS systems are particularly effective in areas where outages are rare, but power fluctuations are common.

The high start-up (inrush) current of oxygen concentrators renders many of the common off the shelf systems, typically used for electrical items such as TVs, incompatible with oxygen concentrators. Much more specialist higher power over-specified UPS systems are required. However, these systems can be expensive. When selecting a device, it is important that the UPS explicitly says that it is designed for inductive loads, otherwise the rated wattage will be incorrect, most are designed for switched-mode supplies.

In addition, once a suitable device has been selected, it is important to consider the batteries that are used,which have a finite life span and must be replaced when they degrade. Lead acid batteries have a life span of three years, or typically 200–300 charge discharge cycles, whichever comes first. It is also important to note the lifespan of lead acid batteries is related to the depth of discharge of the battery cycle, most lifetimes are rated to no more than 50% battery depletion. Lithium batteries, with a higher upfront cost, may last five-ten years and have a greater number of charge/discharge cycles. A good option would be lithium-iron-phosphate batteries (LiFePO4), which are the safest mainstream lithium-ion batteries and have a practical depth of discharge of 80%.

A UPS is one way in which you can ensure the supply of oxygen continues to flow during short periods of power outage or instability, but there are various other approaches, such as low and medium pressure oxygen stores, but these are beyond the scope of this review. Further details are available through the Oxygen CoLab if you are interested to learn more please do get in touch.

Give me some guidance of what is best for my situation

Here are some different scenarios to help you think through what solution is best for you.

Good power quality, occasional infrequent issues, not reliant on the oxygen concentrator as a primary oxygen supply

It may be wise to add surge protection to your concentrators, these are very low cost and would protect your system in the unlikely event of a power surge (e.g. due to an issue on the grid). In this situation it is probably unnecessary to invest in other equipment.

Occasional sags and surges and short brownouts

You should fit a surge protector, and, if your budget is limited at least fit an over and under voltage protection device with start up delay (this normally comes as a complete package). This would protect your concentrator from possible damage caused by fluctuations in the voltage beyond safe limits. You probably also want to consider installing a voltage stabiliser instead of the under voltage protection device as this will allow your concentrator to work over a much wider range of voltage inputs, but this will depend on your budget. If maintaining a continuous supply of oxygen is important, depending on the length of time that you might also want to consider installing an offline UPS to cover short duration brownouts.

Frequent Sags and surges with longer brownouts where you are reliant on the oxygen concentrator

It is essential that you fit good quality surge protection with a high joule rating along with a voltage stabiliser. If a stabiliser is not available, it is essential that you fit an over and under voltage protection device. If your budget can afford it, you might want to consider an online UPS system, as this offers a very high level of protection for your oxygen concentrator. For example, FREO2 has developed a system known as PROTECT designed specifically for oxygen concentrators. The PROTECT system conditions the power to meet the specific requirements of oxygen concentrators. Functions include voltage stabilisation for inputs between 180 VAC to 260 VAC, automatic shut-down outside that range, and controlled restarts to avoid strain on the compressor created by rapid power cycling.

Very poor power quality, with frequent extended blackouts and surges and sags.

An online UPS is essential if you are going to get any meaningful use out of our concentrator. You might want to think about fitting solar power as backup and powering the concentrator directly from solar, when possible, as well as oxygen storage, to provide you with oxygen when the power goes down for an extended period.

Brushless DC motor compressors

Many of these challenges are avoided by using a DC compressor, with an AC-DC regulator. Modern AC-DC regulators manage sags and surges and can take a wide ranging input including variable mains frequency. They are able to precisely regulate DC output. There is also less loss in power loss due to conversion.

Thank you to David Peak at FREO2, Gerry Douglas from OpenO2 and Robert Neighbour from Diamedica for all their input and support in compiling this article.

Thanks to you too for reading this far! For further support in protecting your oxygen concentrator please do get in touch with the Oxygen Colab. If you have any comments, additions, corrections or want further clarifications, we’d love to hear from you!

Glossary of terms

Grid connected electricity: Grid electricity is a network of physical wires for delivery electricity from producers to consumers. It includes power generation (e.g. coal, gas/oil power stations, nuclear power, solar or wind farms, hydro-electric dams etc). High voltage transmission lines carry power over long distances before being converted by a transformer to levels for consumer distribution. Problems with any segment of the system can lead to power issues and outages.

Nominal Voltage and Frequency: The expected values of voltage and frequency that would be measured at an electrical socket. The nominal grid voltage in Nigeria is 230, while in Kenya it is 240. In both countries the power system frequency is 50 Hz +/- 1.25 Hz

Generators: Generators use petrol, diesel or gas to run a motor to generate electricity. They are often used as a backup when frequent blackouts occur. However, poor quality and poorly maintained generators are known to produce voltage spikes, which might potentially damage an oxygen concentrator.

Off-grid renewable energy: Energy can be provided from renewable sources such as wind and solar. However, supply is intermittent and energy storage is required to run appliances continuously.

Over-voltage: A period when the RMS mains voltage increases to more than 110% of its nominal value for more than 1 continuous minute.

Voltage swell/Surge: Short duration (milliseconds, seconds, less than 1 continuous minute) rise in the voltage above acceptable limits. Depending on the level of the over-voltage, the damage can be instantaneous, severe and irreparable. This can be caused by lightning as well as malfunctions in the grid. It is not uncommon for it to occur with the return of mains supply after power cut, or when generators kick in as back up power.

Spikes: Very short, event of very high surge in voltage to thousands of volts and amps. Spikes are common in low resource settings and repeated exposure will damage electronic equipment. It can be caused by switching on/off equipment nearby. Lightning strikes, restarting the grid when a blackout has occurred. Also is common to occur when using poorly maintained generators.

Voltage Sag: period when the RMS mains voltage decreases to less than 90% of its nominal value for less than 1 continuous minute.

Under-voltage: When the RMS mains voltage drops below 90% of its nominal value for more than 1 continuous minute. Long duration periods of undervoltage (minutes to hours or days) are very common in low resource settings where the power utilities are overstretched. Frequent or prolonged brownouts can cause the equipment to malfunction and fail. Repeated episodes are certain to cause damage. Motors and compressors and therefore fridges, freezers, coolers, air-conditioners, pumps and oxygen concentrators are especially at risk. It is common in dry seasons where water is used for electricity generation.

Black outs/Power-cuts: Common in low resource settings, especially in areas of frequent voltage problems. Results in a sudden loss of power. It can be commonly caused by substation failure, breakdown in the distribution network.

Power-back surges: These typically occur when power returns after a power-cut and connected equipment receives a surge of electricity at an over-voltage level, which can be very damaging (see above). The cause can be power companies restoring supply at an above normal voltage in order to compensate for the demand of connected equipment which will re-starts simultaneously.

Root Mean Square (RMS) Voltage: The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. It continually varies between a max and a minimum in a sinusoidal waveform. The RMS voltage is an average that provides the effective value of a varying voltage. It is the equivalent steady DC (constant) value which would provide the same power.

RFI (radio frequency interference)/noise: High frequency disturbances that occur within a short period of time (milliseconds). RFI & noise are very common low resource settings and are the main cause of data corruption. What causes it? Generated by high frequency noise from nearby equipment like TV, radio equipment, transmitters, mobile phones, switching on/off of certain loads, fluorescent lights, motor speed controls, light dimmers. Although we do not discuss this in the article, this is something to be aware of.