Inrush current - a hidden safety problem

Published:  17 December, 2019

Alarm sounders and sounder beacons are safety-critical notification devices present in building procedures and manufacturing processes across the globe, so you need to be aware that there is a hidden issue that could cause problems resulting in unwanted plant downtime. James Morgan, product manager at Eaton, takes a closer look at how to mitigate or prevent the effects of inrush current.

Health and safety is at the top of any plant manager’s agenda, so if there is a problem then equipment or plant is rightly shutdown until you can rectify it.

Against this backdrop there is a hidden problem that can cause safety critical alarms to stop working, which, given the right information for better system specification, is entirely preventable.

The problem is caused by the inrush current needed by alarm devices on start up. This generates a high initial current spike that only occurs for a very short period of time but its affects can be serious.

For these devices, it is the capacitance within the product that causes this inrush current. The capacitators provide a large reservoir of energy to smooth out the current draw that the device needs to produce a sound loud enough that people can hear in a noisy environment or over a large area and/or provide a flashing beacon as a visual alert. When the device is switched on these capacitors draw current rapidly to fill up this energy reserve (see diagram 1 blue line). This initial current can be between 50 to 100 times larger than the operating current.

If there are only one or two devices on the circuit, this load will only have a small effect on the system as a whole, but it could still damage switch and product rectifier components.

The problem becomes more serious when there are multiple devices on a line starting up simultaneously. The sum of the inrush currents from all these devices can cause a voltage drop on the line delaying their start or even blow a fuse or trip the RCD. If equipment is on the same line as the devices then it will also be affected, but regardless of this if the sounders are not working then there is a critical health and safety issue that needs resolving.

This issue is even worse in low voltage DC systems that have long cable runs and /or minimum gauge cabling where the increased resistance causes further voltage drops.

Yet we find that such notification devices are typically on low voltage DC lines routed along existing conduits. In fairness there is a good reason for this, since by using low voltage lines you do not need qualified electricians to do the maintenance. It also means that you don’t need to isolate the circuit and therefore switch off any other plant or equipment on that line in order to work on it.

Demand the information

Why do I call inrush current a hidden problem? Well for one thing it is generally not stated on a product or even a product datasheet. Typically, you will find details about the operating current, which is the current required after the device has started up, but there is generally nothing mentioned about that initial inrush current.

So, the first thing to do is ask your supplier what the inrush current is for its sounder or sounder beacon, especially if you intend to install a number of devices on a low voltage line.

Without this figure it is extremely difficult to calculate the supply componentry and power supply needed. Get this wrong and there is a risk of current overload and the devices not working.

Another tactic to avoid the cumulative affects of several devices switching on simultaneously is to stage a progressive switch on of products over a short period of time. If a PLC controls the plant, you may find that it switches everything on at the same time making this impossible.

Alternatively, you can over specify the power supply, switchgear and cabling. At face value you have now dealt with the issue, but at a cost. You end up with an over-engineered solution with unnecessary cost by using bigger cables or transformers that you don’t need for everyday operation.

Minimising inrush

Faced with such a conundrum it is worth exploring what lessons you can learn from other applications of notification devices. In this case I’m referring to fire detection, which also warn people about a hazard.

On a typical fire detection system in a large commercial building there will be several notification devices on a 24dc line that could be several kilometres long. It is a heavily regulated market so you cannot increase the voltage or add additional equipment, such as transformers, into the system.

This means that any startup surge current can cause serious voltage drops on the system, so the devices do not start up at all or, because the output lines from the fire panel are fused, the sum of this initial surge current from several devices can blow this fuse.

The answer is to manage the inrush current over a longer period of time to reduce any spikes. There are several ways of managing this inrush current for alarm devices, at Eaton for example our engineers developed patent pending current clamping technology for our X10 industrial sounders and sounder beacons. The result is less voltage drop and a far lower chance of a circuit breaker tripping or the fuse blowing (see diagram 1 red line).

Such solutions mean that you will need to purchase slightly more expensive devices, because of the componentry needed to achieve this; but in relation to the cost of over engineering an entire system or stopping preventable downtime, this pales into insignificance.

Of course, not every application will require you to install devices with this capability. If you are using a mains voltage line, or only require one or two devices then there may be nothing to worry about. The problem is that for too long the issue has been hidden.

Surely it’s time for manufacturers to clearly quote not only the operating current on sounders and sounder beacons but also their startup current. Given all the information that you need, you can then design a cost-effective notification system that will avoid downtime and keep your personnel and equipment safe.

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