安全栅防爆元件的应用

工业上的酒精、汽油等可燃物品的生产和使用都涉及到防爆技术。常用的防爆技术包括:隔爆、本质安全、油浸、充砂、浇封等。在电气防爆中,本质安全是使用最多的技术,它要用到一个关键元件——安全栅。本文包括如下两个主题:安全栅的作用及分类;

1、安全栅的作用及分类在电气本质安全防爆(简称:本安防爆)中,一般我们把工作场所分为防爆区域和非防爆区域。防爆区域和非防爆区域之间有电能的传输,比如,防爆区域的开关、按钮、传感器等通过电线与放置在非防爆区域电气柜中的PLC相连接。安全栅的作用就是在防爆区和非防爆区之间起到能量限流的作用,它降低防爆区域内信号的电能,从而降低其产生电火花的风险。安全栅根据结构的不同,分为齐纳式安全栅(Zener Safety barrier)和隔离式安全栅(Isolated Safety barrier)两类。齐纳式安全栅在其内部电路中串联快速熔断丝、限流电阻和并联限压齐纳二极管,从而实现限制能量的作用,它具有如下一些特点:①必须有单独的、非常可靠的接地系统(本安系统接地),接地电阻应小于1Ω;

②不能进行电信号转换,对于热电偶/热电阻(RTD)等信号可能会由于电磁干扰而出现错误,因此不适合连接该类型的传感器;

③对供电电源电压影响非常大,电源电压的波动可能会引起齐纳二极管的电流泄漏,从而引起信号的误差或者发出错误电平,严重时会使快速保险丝烧断而永久损坏。按规定齐纳式安全栅内部齐纳管、限流电阻、保险丝整体浇封,一旦损坏无法修复;

④体积小,价格便宜,不需要额外供电;

隔离式安全栅的输入、输出及电源三者之间相互隔离,它具有如下一些特点:

①不需要接地线,现场施工非常方便;

②由于信号线无需共地,使得整个系统的稳定性和抗干扰能力大大增强;

③具有信号转换能力,可以处理热电偶/热电阻/频率等信号,比如将其转换成4~20mA的电流信号输出,抗干扰能力比较强;

④一般隔离式安全栅具有两路输出通道,相互之间电气隔离,互不影响;

⑤价格相对贵些,需要外部供电;

虽然隔离式安全栅价格相对较贵,但其出色的性能和便于施工的特点,使得在工业现场有广泛的应用。接下来我们介绍下倍加福(P+F)公司的一款隔离式安全栅产品。

2、倍加福隔离式安全栅介绍

下面以此品牌为例,倍加福(P+F)的KFD2-ST2-Ex2隔离式安全栅用于开关量信号输入,其外观如下图所示:

它有两路隔离式通道,可以连接干接点开关信号,需要外部24V电源供电。模块上、下各有两个接线端子,其中:

引脚1、2、3和7为通道1;引脚4、5、6和9为通道2;14、15为24V电源;

接线原理图如下图所示:

以通道1的使用为例,接线时将引脚1和3连接防爆区域的开关,将引脚7连接到非防爆区域的PLC输入通道,14接24V电源正极,15接负极。好了,关于安全栅就介绍到这里。

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