SURGE & CIRCUIT PROTECTION FEATURE


Inrush current - a hidden problem H


In this article, James Morgan, product manager at Eaton, takes a closer look at how to prevent the effects of inrush current


ealth and safety is at the top of the agenda for any industrial


environment, so if there is a problem, the equipment or plant is rightly shutdown until it can be rectified. Yet there is a hidden issue that can


cause safety critical alarm devices to stop working, which is entirely preventable given the right information for better electrical system specification. 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 effects can be serious. For these devices, it is the capacitance


within the product that causes this inrush current surge. The capacitors 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 surge can be 50 to 100 times larger than the operating current. The problem becomes serious when


there are multiple devices on a long low voltage 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 blowing a fuse or tripping the RCD. Yet, we find that such notification


devices are typically on low voltage DC lines routed along existing conduits to make maintenance easier. The problem is that while information


can be found about a device’s operating current, there is typically nothing stated about this initial inrush current, making it hard to calculate the supply componentry and power supply needed. The issue can be mitigated by staging a


progressive switching on of products over a short period of time, although often a PLC controller will automatically switch everything on at the same time. Alternatively, the power supply, switchgear and cabling, can be over specified, but this is an over-engineered and costly solution. There is however another answer. Eaton decided that it was time to apply


the lessons that it learnt from the fire detection industry. On a typical fire detection system in a


large commercial building there will be several notification devices on a 24 dc line that could be several kilometres long.


the voltage can not be increased or additional equipment such as transformers added into the system.


This means that any start up 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


It is a heavily regulated market, so


current over a longer period of time to reduce any spikes. Applying this lesson to industrial notification devices, Eaton engineers developed patent pending current clamping technology for its X10 devices. 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 an answer does give a slightly


more expensive device, 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. Not every application will require devices


to be installed with this capability. If a mains voltage line is being used, or only one or two devices are required, then there may be nothing to worry about. The problem is that it is a hidden issue. At the very least it is time for


manufacturers to clearly quote not only the operating current on notification devices, but also their startup current. Given this information, a cost-effective notification system can be designed that will not cause downtime, and keep personnel and equipment safe.


Eaton  eaton.uk.com ELECTRICAL ENGINEERING | NOVEMBER 2019 21


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